When looking at examples of community, as community, it is probably a good idea to look for other types of communities as well, and identify common factors.

One other, successful type of community (which, like the desired community, is volunteer-run, and consists of people with a shared goal of self-improvement) is Toastmasters. The self-improvement in question is in the narrow realms of communication (especially as regards speeches) and leadership, but a lot of the basic principles would probably carry over reasonably well to a rationalist community.