This is an inverse value to scale question. Think about your Rolodex. A thousand contacts, maybe 150 people you can call friends, 30 people you can call close friends, two or three people you'd donate a kidney to. The value is inverse to the size of the group. And you have to find some way to protect the group within the context of those effects.
Sometimes you can do soft forking. Live Journal does the best soft forking of any software I've ever seen, where the concepts of "you" and "your group" are pretty much intertwingled. The average size of a Live Journal group is about a dozen people. And the median size is around five.
But each user is a little bit connected to other such clusters, through their friends, and so while the clusters are real, they're not completely bounded -- there's a soft overlap which means that though most users participate in small groups, most of the half-million LiveJournal users are connected to one another through some short chain. —