In 1978, several hundred people in Jonestown drank poison and died, because their leader told them to. Not everyone who died there was a volunteer; some members of the group objected, and they were killed first by the more dedicated members, who killed themselves later. Also, many children were killed by their parents. Still, hundreds of people died voluntarily, including the leader of the group, Jim Jones.
This is an extreme case. There are much more groups that create a comparable level of devotion in their members, but use it to extract money and services from the members. Groups where new members change their personalities, becoming almost "clones" of their leaders; then they break contacts with their families and former friends, usually after trying to recruit them for the group, or at least trying to extract from them as many resources as possible for the group. The new personality typically has black-and-white thinking, responds to questions by thought-terminating clichés, and doesn't care much about things unrelated to the group. Sometimes the membership in the group is long-term, but commonly the members are worn-out and leave the group after a few years, replaced by the fresh members they helped to recruit, so despite the individuals change, the group remains.
Talking about cults is difficult. It is not simple to provide a definition -- in different contexts finding new friends and growing apart with the old ones, spending money on hobbies or causes, and gradual changes in personality could be a part of normal life. Even if this all happens much faster and more extremely, is it just a question of degree (and how much exactly is too much?), or is there something else?
Of course, a group accused of being a "cult" will object against the label and point out the differences between them and some archetypal cult they are being compared with; or insist that there are no cults, only people intolerant of different lifestyles. They will emphasise that their members are in the group voluntarily, so no one else has a right to decide what would be better for them. And for every strange norm or behavior, there is always a good explanation (or a denial).
On the other hand, people will use the label to call any group they dislike a "cult". Some of them will use it as a synonym for "a religion other than my own" or "people who have a weird hobby". And of course any objection will be met by: "And that's exactly what a cult would say." (Which could actually be true, but that does not exclude the possibility that both sides focus on a wrong thing.)
In absence of clearer criteria of what constitutes a "cult", or more precisely, what kind of personality change by group influence is that kind of change we worry about... what kind of influence can change normal people into blind servants of the group without any obvious coercion... the debate will be very confused.
Also, for people who don't have an experience of being a cult member, it may be difficult to imagine what is it like. Many people imagine that being in a cult is a thing that can only happen to other -- read: stupid and gullible -- people; they underestimate the emotional forces involved. (Ironically, if they later become cult members themselves, the same belief will make them argue that their group is not really a cult.) But even the former cult members may have a problem to pinpoint the essence of the problem, instead of focusing on some accidental attribute of their former group. It is not rare that former cult members join a different cult later.
Luckily for our debate, there are relatively objective criteria for the cultish form of "mind control". There are a few "red flags" you can learn to notice and evaluate. The list called "eight criteria of thought reform" was compiled by Robert Jay Lifton, an American psychiatrist who studied "brainwashing" of prisoners of war, and indoctrination in totalitarian regimes.
As a rule of thumb, it is better to ignore the specific beliefs of the group, and focus on its behavior. Debating the beliefs is a red herring. There could be two groups worshiping the same sacred scripture, and yet one of them would exhibit the dramatic changes in its members, white the other would be just another mainstream faith with boring compartmentalizing believers; so the difference is clearly not the scripture itself. There could also be two groups with nominally different beliefs, but otherwise strikingly similar behavior. The beliefs don't even have to be religious per se; the group can also believe in a political reform, or an economical success in the latest pyramid scheme.
It is best to pretend to be an alien zoologist, studying the homo sapiens species, unconcerned with object-level human beliefs. Notice when the members wear the same dress, but ignore what specific dress it is. Notice when the members are required to repeat specific words every day, publicly or privately, but ignore what those words mean. Notice when the members consistently behave remarkably friendly to a new recruit, and avoid contact with former members, but ignore their explanations why exactly are they doing that. Et cetera. Observe what they are doing, not what they believe they are doing.
Sometimes a group matches a given criterion partially. Sometimes the group matches some of the criteria, but not all of them. There is no exact line unambiguously separating "cults" from "not cults". But that doesn't make the concept worthless. There are groups that exhibit the typical cult dynamic, with all the toxic aspects reinforcing each other. There are also groups that have some unhealthy behavior, but it doesn't create the full pattern. Even the same group can change over time. To evaluate a given criterion, it is good to have an idea how far does the scale go in a typical cult (to avoid the "medical student syndrome" when people find every symptom in everything). Probably the best way to get the idea is to read a few biographies of the former cult members.
So, here are the criteria:
Living in a controlled environment
There are rules for how you should spend your time. Some groups insist on wearing a uniform; some groups want you to spend the whole day in presence of at least one other group member, to make sure you don't privately break the rules. Sometimes there is a lot of work to keep you busy -- whether real work, or just repeating a mantra thousand times a day, -- so you don't have free time to reflect on the group and your membership. (People sometimes spend months or years in a cult without actually making a conscious decision to join it permanently; they just decide to hang out with them for a while to learn more about them, and receive a ton of "urgent" work with a promise that all their questions will be answered later, when there is more time. Somehow, that never happens, and only after a few months or years these people realize they were duped and leave.)
More important than controlling the physical space is controlling the mental space. All groups agree that you should avoid reading materials critical of the group, and talking with people critical of the group (unless you are trying to convert them; or you are exposed to them strategically, because they happen to be stupid critics, so the real goal of exposure is to teach you that this is what all group critics look like). Most of all, you must avoid talking with the former members (they know too much).
Of course, this is all for your own good. People opposing the group are the worst people of all; it is rational to avoid spending your precious time with them. They are wicked heretics, slaves to sin. They are racists, sexists, capitalists, or communists; the evil ones who keep ruining our society. They are brainwashed sheep who are too scared to think out of the box, join our latest pyramid scheme and make millions. The former members were too sinful or weak to stay in the group; you better avoid them, because the weakness might be contagious; and don't believe anything they say!
This also applies to your internal environment. If there is a voice in your head telling you to slow down, to take an outside view, to consider the alternatives... you are supposed to fight against that voice, not listen to it.
Sometimes the protection against outside information happens on two levels. There is information that the group will officially admit they are trying to protect you from, such as pornography or harassment. They may offer to install a group-sanctioned web filter, or otherwise let you outsource the information filtering to them. If you let them, you usually get more than you subscribed for. For example, the anti-pornography or anti-harassment filter will also filter out information and people critical of the group, without telling you it did so. (Now good luck trying to convice your employer or university to remove those filters again.)
Guided by an intelligence higher than you
There is something special about this group and its leadership, which makes them unique in the universe and in the history. If you tried to make another group like this, it just wouldn't be the same. You couldn't even change it significantly, because it was designed exactly the way it needs to be.
If this is a religious group, the answer is obvious: it is the supernatural power on your side that makes the difference. But even nominally non-religious groups often believe in things that are supernatural in anything but name: the inevitable flow of history, the spirit of time. The non-religious leaders may nominally be mere mortals like you, but they are mortals endowed with unique understanding of the forces of the universe, which makes them incomparable to you; you couldn't replicate (or surpass) their wisdom no matter how much would you study. Similarly, you couldn't devise your own pyramid scheme, or invent a better product to sell. It is insane to even think about such things.
When your group is guided by a higher intelligence, the only rational thing to do is to obey it without a doubt. Even weird things that happen (for example, one day your leaders predict the end of the world, the next day they deny doing so), happened for a purpose; you are just too stupid to understand it.
What do we want? Perfection! When do we want it? Now!
The perfection is not only achievable right now, but you are a horrible person for not having achieved it already. What stops you? Everything is either completely good or completely bad; just throw those bad things away! Unless you secretly prefer to be evil.
(Of course, deep inside you will always be aware of your own imperfection, as measured by the group standards. Just like every other member of the group. But that's not a bug, that's a feature! Feeling guilty will make you work harder.)
The same black and white standard is also applied to everything outside the group, usually by finding a flaw, and concluding it is too horrible to interact with. It creates a perfect excuse to avoid anything or anyone inconvenient for the group leadership. It also guarantees a steady flow of perceived enemies, because no microaggression is too small to make a mountain of.
Your mind belongs to the group
There is no privacy even inside your head. You are supposed to confess your sins to the group. (Note: There is always something to confess. Believing you have nothing to confess is itself a great sin.) Confession is best done publicly, in front of the whole group. If you don't volunteer enough sins, the group is supposed to call you out. Snitching on each other is a valuable spiritual service to your fellow members. The most poweful confessions are ones that leave the victim broken and in tears. Then the victim swears to sin no more, and the group provides an absolution. This creates a strong emotional bond between members.
A sacred science
The teaching of the group come with an authority of science. Except, this is a special kind of science; one that does not allow doubts. The truth is not determined by experimentally comparing competing hypotheses; it is revealed. The leaders of the group got everything right for the first time. Their work is perfect. There are no alternatives to consider. Skepticism is evil.
Redefining the language
You can either invent new words, or redefine the meaning of the existing ones. This serves multiple purposes. On the outside, it creates yet another barrier between members and non-members. It is impossible to get a second opinion on group teachings, if non-members don't even understand what those words mean. (And if they don't understand, that obviously makes them less smart, right? More words mean more wisdom.) Every moment a member spends with non-members, these artificial barriers in communication keep reminding them they are in the outside world. The group becomes the only place where some concepts can be discussed meaningfully.
Redefining the language also brings specific advantages in teaching the group beliefs. You don't really have to prove anything if you simply make it a part of the definition. For example, if you redefine "heresy" to mean "irrational disagreement with the beliefs of our group", you don't have to prove that people who disagree with your beliefs are irrational. It's right there, in the definition! Or you can redefine words "X" and "Y" to mean "X, but only when our members do it" and "Y, but only when non-members do it", and suddenly it is obvious that when both members and non-members seemingly do the same thing, it's actually not the same thing.
Short phrases can be used to stop thinking about complex topics.
Map over the territory
The group doctrine says something can't happen. But it happened to you, maybe years ago, maybe right now. Guess what: You are wrong! It didn't happen. And you better change your memories; either find an interpretation that fits the group teachings, or just deny the whole thing. Because the group is always right.
Often, among the memories that need to go, is anything good that happened to you before you joined the group. The official story usually requires you to tell (and believe) than you were a horrible and worthless person before you have seen the light.
If it's not here, it's not real
The worlds inside the group and outside the group can't really be compared, because one of them is not even real. People inside the group are saved. People outside the group are doomed, and they deserve it; they are evil and weak. The people outside are only valuable as potential future members.
This of course creates the threat that if one day you would lose the group membership, you would stop existing, too. Not literally disappear, just stop existing as someone who matters.
The important thing here is to realize that these eight points aren't just eight arbitrarily selected things, but rather eight aspects of the same complex mechanism that hacks the human brain; makes it value the group highly, makes one feel dependent on the group, and builds a barrier of fear against leaving the group. They often reinforce each other: The argument by higher intelligence can be used to explain why the rules exist; the demand for perfect purity includes requiring perfect obedience of the rules; the group confession is there to detect transgressions; and the controlled environment even removes the opportunity for transgressions. Similarly, black and white thinking classifies your former friends as evil; the group confession (where you must report "sinful" topics you talked about, such as your former hobbies, or how the group membership keeps changing you for worse) makes meeting them more costly; the new language makes communication more awkward; rewriting your memories requires you to give up good memories experienced with them; and pretending that the world outside the group is not fully real makes you rationalize that nothing of value was lost.
When people without their own personal experience talk about cult membership, they sometimes describe it as a rational preference for things offered by the group to its members, such as the social environment or the feeling of meaning. While this contains a grain of truth (a group offering its members literally nothing probably wouldn't be popular), focusing on the value provided by the group ignores the more important part of the picture: how the members are strategically deprived of potential value coming from all alternative sources, so the group becomes a monopoly on value, threatening to take it all away as a punishment for disobedience. Of course, surrendering to the group demands only makes one more vulnerable in long term.
How do such well-designed abusive groups appear at the first place? I believe a part of the answer is selection bias: abusive groups with worse designs remain small or gradually disappear. Other part is that some of these techniques come naturally to abusers (such as: isolating the victim from their family and friends, or accusing the victim of hundred made-up "crimes" so the debate shifts to talking about what the victim did wrong), and the only technical problem is building the group structure that will make other members play along with the abuse (especially considering that they will also receive the same treatment). And yet another part of the explanation is that successful groups copy these "best practices" from each other, because they see greater group cohesion as a worthy subgoal. (Here is a nice video.)
So, what can one do to avoid a similar outcome? Some people suggest full reverse stupidity: a sufficiently safe group should consider itself and its members completely worthless, should avoid having any specific opinion on anything, and preferably shouldn't even exist. Anything else would be the first step towards the inevitable horrible outcome. But looking at the eight criteria, I believe there are ways to have a functional, yet non-abusive group, if we simply avoid scoring too high at any of these points.
A surprisingly large part of this could be classified as some aspect of "free speech" -- being able to talk to anyone (even critics of the group, and former members), about anything (even doubting the group beliefs, debating personal experience that contradicts the group beliefs, expressing doubt in ways the supposed higher intelligence guides the group, and explaining group jargon to outsiders), and keeping the content of such debates for yourself. The rest could be described as quantitative thinking (accepting that some things can be better or worse than others, without that inevitably making them perfect or horrible), and cooperating with people outside the group. As a rule of thumb, a group that makes fun of free speech is an abusive group; some of those things you are not supposed to discuss are about how they treat people, including their own members. On the other hand, a problem you can talk about is a problem you can try to solve.