I'm painfully familiar with the issue of lack of group participation, since I can't even get people to show up to a meetup.
Because of that, I've been doing research on identifying the factors contributing towards this issue and how to possibly mitigate them. I'm not sure if any of this will be new to you, but it might spark more discussion.
These are the first ideas that come to mind:
1. For people to be intrinsically motivated to do something, the process of working on it has to be fun or fulfilling.
2. Extrinsic motivation, as you say, requires either money or a reason to believe the effort will accomplish more than other uses of one's time would. If it's a long-term project, the problem of hyperbolic discounting may lead people to watch TV or [insert procrastination technique here] instead, even if they think the project is likely to succeed.
3. If people already have a habit of performing an activity, then it takes less effort for them to participate in similar activities and they demand less benefit from doing so. Identifying habits that are useful for the task you have in mind can be tricky if it's a complex issue, but successfully doing so can keep productivity consistent and reduce its mental cost.
4. Building a habit requires either intense and consistent motivation, or very small steps that build confidence. Again, though, identifying very small steps that still make for good productivity early on may be tricky.
5. If you have trouble getting people to start joining, it may be good to seek out early adopters to provide social proof. However, the social proof may only work for people who are familiar with those specific early adopters and who take cues from them. In that case, you may need to find some regular early adopters and then identify trendsetters in society (habitual early adopters from whom many people take their cues) you could get on board, after which their followers will consider participating. (Then the danger becomes making sure that the participants understand the project, but at least you have more people to choose from.)
6. It may help to remind people from time to time what they're working towards, even though everyone already knows. Being able to celebrate successes and take some time to review the vision can go quite a ways in relieving stress when people start to feel like their work isn't rewarding.
From item 1, if people think they can get a benefit from working on a project even if the project fails, they might be willing to participate. Socializing with project members and forming personal relationships with them may help in this respect, since they'll enjoy working with people. Alternatively, you could emphasize the skills they'll pick up along the way.
From item 4, I've been working on 'mini-habits" (a concept I got from Stephen Guise) to lower my own mental costs for doing things, and it seems to be working fairly well. Then the trick becomes getting enough buy-in (per item 5) so you can get other people started on those mini-habits.
There are probably some other factors I'm overlooking at the moment. Since I haven't been able to get results yet, I can't say for sure what will work, but I hope this provides a helpful starting point for framing the problem.
(Cross-posted from Facebook.)
I've noticed that, by my standards and on an Eliezeromorphic metric, most people seem to require catastrophically high levels of faith in what they're doing in order to stick to it. By this I mean that they would not have stuck to writing the Sequences or HPMOR or working on AGI alignment past the first few months of real difficulty, without assigning odds in the vicinity of 10x what I started out assigning that the project would work. And this is not a kind of estimate you can get via good epistemology.
I mean, you can legit estimate 100x higher odds of success than the Modest and the Outside Viewers think you can possibly assign to "writing the most popular HP fanfiction on the planet out of a million contenders on your first try at published long-form fiction or Harry Potter, using a theme of Harry being a rationalist despite there being no evidence of demand for this" blah blah et Modest cetera. Because in fact Modesty flat-out doesn't work as metacognition. You might as well be reading sheep entrails in whatever way supports your sense of social licensing to accomplish things.
But you can't get numbers in the range of what I estimate to be something like 70% as the required threshold before people will carry on through bad times. "It might not work" is enough to force them to make a great effort to continue past that 30% failure probability. It's not good decision theory but it seems to be how people actually work on group projects where they are not personally madly driven to accomplish the thing.
I don't want to have to artificially cheerlead people every time I want to cooperate in a serious, real, extended shot at accomplishing something. Has anyone ever solved this organizational problem by other means than (a) bad epistemology (b) amazing primate charisma?
EDIT: Guy Srinivasan reminds us that paying people a lot of money to work on an interesting problem is also standardly known to help in producing real perseverance.