Forget Dunbar, you’re not close friends with more than 8 to 13 people in any given community. 8 to 13 is about the number at which groups begin to split. You don’t see 150 kids standing together in a playground, you see smaller groups of 8 to 13. Past 13 people, people begin to be left out.
Don’t focus on “the community” as a whole. The community is a by-product of you working hard to create lots of interconnected groups. If you focus on the mass, you end up with a single dedicated few members, a few fringe members, then the outsiders (lurkers). Without groups, bigger is bad.
So how do you develop groups in a community?
First, you need as many interactions taking place between members as possible. Interactions lead to relationships. Without relationships, there are no groups. You start interactions by initiating stuff. Events, chats, discussions, content. Do everything possible to encourage interactions between members.
Next you identify and encourage the primitive friendship groups which develop. Give them a place to breathe. Take some time to refer to groups by name in your content. Create challenges between groups. Keep score and tallies of their members. Invite them to create a name and logo for themselves.
This is the biggest thing separating expert community builders from mediocre builders. The experts can develop groups within their community.