I'm the de facto organizer of the Austin meetup, which has been meeting weekly for approximately 4 years. It has mostly been socializing at a coffeeshop, eating dinner at a home, or board games, and typical attendance has ranged from 3 to 9. I think a dozen was the most people we ever had at one time outside of the HPMoR wrap party. We have a fairly smooth gradient from regular members (people that show up every week) to people only on the membership list.

My impression is that socialization starts to fall apart at about 4 people, and becomes very unwieldy by 8 (speakers; listeners are basically free, which is why talks and debates work). Outside of a party format (where creation and fragmentation of groups is easily encouraged), though, splitting up doesn't seem to work that well (if you're sitting at two tables at a coffeeshop, say, or having multiple conversations at one table, it's difficult to reconfigure people so that people are close to who they want to talk to, as topics and mutual interests change).

What's stopping you?

The main issue stopping us from doing more is a lack of spare attention and energy, I think. It seems like there's a low effort attractor where people meet up at the same time at the same place and just socialize, and the high effort attractor where there are lots of things going on that cater to many peoples' interests and accomplish things. A recurring question is how much energy to put into main projects and side projects--we could, say, go learn to swing dance as a group, which would have a number of positive effects, but would come at the expense of whatever people would have spent that time on otherwise--which, presumably, would have had their own positive effects. Time I would spend on, say, generating talks for a LW meetup probably competes with time I would spend on, say, writing posts for LW, and it's not clear to me that it's better to do the former than the latter.