Epistemic status: rough partial-model formed from... well, basically a single conversation, but which resonated with several years of vague accumulated impressions. Curious if it resonates or anti-resonates with others.
In the comments of Mingyuan's "Meetups as Institutions for Intellectual Progress", Bendini notes:
The current format is not ideal for accomplishing anything, so much so that I've stepped down from organising mine because they were providing so little value. It's a sad state of affairs, but from what I can tell the majority are content with them being low-effort social groups.
In my experience this is fairly common – most people who show up to meetups are just sorta there to hangout. If you get 20 people, maybe 3 of them will be driven to proactively "do stuff."
I was chatting with a friend recently who said [paraphrased] "Honestly I'd rather go to a social gathering where I can just hang out than one where we're doing a bunch of effortful stuff... but I enjoy those meetups more when they're filled with the sort of people who do do effortful stuff (even if they're not right now). They're more... alive and interesting."
And this matched a couple years experience running the NY meetups – it seemed like what most people wanted, most of the time, was fun social gatherings.
On any given meetup night, most of the value was not in "the presentation on Bayes Theorem" or whatever that gathered people together – it was the chatting afterwards and talking about whatever else was interesting. If you ask someone "would you rather just skip to the part where we hang out and chat, rather than do a workshop or listen to a presentation", many people would say "yes."
Nonetheless... in eras when the NYC meetup swerved towards "mostly social hangouts", attendance dropped, and the vibrance faded. There's a lot to do in NYC and "just hang out" isn't an exciting enough option to compete with many alternatives (including "rousing yourself out of your house in the first place", which requires activation energy).
And I suspect this might be fairly common... at any given level of "amount of effortful stuff you do." 
"Hanging out" is a valuable, important part of a healthy lifestyle. And the phrase "hanging out" implies not trying hard to build a new thing or learn a new skill. If you hang an object out, you're draping it over a clothesline or couch or something and the object is not doing anything effortful.
A model here might be something like:
- People enjoy chill social spaces where they can relax and flow downhill. Do whatever is easy and fun.
- But, when flowing downhill, the amount of "reward points" you get is proportionate to your altitude at the moment of flow. i.e. if you've climbed 20 meters up, you get 20-chill-hangout-points for flowing down for one meter's worth of time. Then 19, for the next one, etc.
- Therefore, even though at any given time one might prefer to chill, a healthy meetup needs to do dedicate time to climbing uphill, in the form of actions that are effortful but build up the overall ambition and interestingness of the social space.
 Footnote that turned out to be an alternate model:
For people naturally motivated to "do stuff", I think there's a weird... "uncanny summit" (contrasted with uncanny valley) of people-who-"do stuff"-in-a-community context.
Romeostevens made a provocative claim in response to Scott Alexander's "What are the Open Problems in Human Rationality?" answer:
I think communities are typically about avoiding responsibility for making personal progress. People who choose to take a more central role in a community typically have emotional problems they are trying to work out via the dynamics in the community. The whole is typically much less than the sum of its parts.
Something about this felt true, although I'm not sure I agree on the terms that Romeo meant to convey. I think what I have to say here is mostly a different model, but felt enough like a response to Romeo that it seemed worth including the link back.
My subjective experience feels less like "Meetups were a way to resolve emotional problems", and more like:
- Meetups were a way to meet basic emotional needs (i.e. feel valued)
- Meetups were a reward signal I could attach to a project that I wanted to do, but was a bit lonely to work on by myself. i.e if I did some research, or thought a bunch of thoughts about a thing, it was nice to be able to give a presentation to a bunch of people and have them say "thanks!" at the end, rather than have it just sit in my mind or bring it up casually.
- Meetups were a source of... well, guinea pigs for random social experiments.
(I imagine Romeo looking at all of this through a different lens, part of which involves detaching from the particular social reward structure at play here, but I don't have a good enough sense of where he was coming from to respond within his frame)
The Uncanny Summit of Meetups
If you have zero people-who-do-stuff, you have a chill hangout space that lasts as long as it provides social value to the people involved.
If you have a few "do-ers", whose projects and excitements aren't directly synergistic with each other, you have a space where do-ers show up and provide excitement and value, which acts as a social reward for the do-er doing their thing, and/or providing guinea-pigs if the do-er likes running social experiments.
This works so long as you have do-ers who are motivated to do things at "meetup scale", who get social reward from the group. (This can last indefinitely depending on the do-er's goals and whether they're legit friends with people in the group, which depends in turn on social chemistry)
If you have a critical mass of do-ers who are interesting in the same thing, you have a brief window where it makes sense for the do-ers to do things together through the community. The community serves the sort of role that I think a lot of people come away from The Craft and the Community sequence expecting.
But frequently, do-ers end up with ambitions for their Things to Do that are larger or more intense than meetup-scale, which lead to forming orgs or teams, or even just solo projects that just aren't really amenable to "present your thing at the local group." And these don't fit neatly into the "community that you can easily join" bucket. They require actually hiring people, which is a different sort of thing.
I have some sense that some people (including myself) had a particular vision of meetups that actually only works in a narrow set of circumstances. And rather than thinking of meetups as a place where Good Work Gets Done, it's better to think of meetups as part of an ecosystem where people flow, depending on where they are in life and what their goals are.
 I don't think Eliezer actually meant people to come away from The Craft and the Community expecting community, in the way that some people (myself included) have tacitly assumed.