Interlinking Makes Communities
A lot of mini-communities have a strong leader, but limited relationships between the members. The group (supposedly) meets weekly for drinks/board games/sports/dinner, but only one person actually organizes it, and if she stops organizing (temporarily or permanently) the group stops meeting. In some sense I'm not even sure this is a community, or if it's just Friends of The Organizer masquerading as a community: if the others in the group would never meet up with each other separate from the host, all you've really got is a hub and spokes.
Other mini-communities are fully decentralized and have no leadership. My experience with these has been that they either 1) meet incredibly rarely, or 2) turn into a Strong Leader community eventually. The goal of a democratic, self-organising group feels admirable, but in practice it seems like if nobody specific is in charge then nobody does anything at all.
I was once part of an interesting online thingumajig that matched you every week with one other group-member for a video chat. I was skeptical going in that this would be fun at all, because what kind of boor/bore would be interested in talking to a random stranger every week? To my surprise, I enjoyed most of the conversations and met many interesting people. However, I wouldn't really say it was a community, since I didn't meaningfully feel like I was part of a group, so much as a long series of (fun) 1-on-1s.
Which makes me sad, because presumably many of the people I met on the platform had also already met each other through it, and the same people I enjoyed meeting probably enjoyed each other too. And I think that points at an interesting alternative way to build communities: building them up via 1:1 relationships.
You can imagine a project like the online matching thing above, but which assiduously matches individual members in a dense set of 1:1s, then (once people have met each other) bundles them into 3s and 4s and 5s: pretty soon you have a friend-group.
Or you can imagine this as a better way to build real life communities: instead of just hosting group events, you match-make individual members with each other until everyone has at least a few strong 1:1 ties within the broader collective. Then the community stops just being Jo And Friends, and becomes A Bunch Of Interlinking Friends In A Dense Network. I suspect that this kind of network is much more likely to create a sustainable social community.
My friend D., a noted expert on group-chats, once told me that "a group chat is only as strong as the weakest 1:1 connection within it." I don't think this maps exactly to larger communities – it can't be true that a community requires every person in it to have a strong 1:1 link – but some weaker version probably holds.
Maybe there's a mathematical formula to it, and if I ever get less lazy maybe I'll look for it: maybe good communities happen when everyone in the group has a true 1:1 connection with at least n others in the group, or 1/m of the total members, or (p/q^2), or some other such thing. Whatever it is, I suspect that this is where many (modern urban secular etc) communities go wrong, and how better communities might start.