They're about as insular as the Unitarian church or Hacker News. It's basically an online community where anyone can check in, check out, and leave a comment or an essay. There's no formal structure or membership organization. The people who write or associate there could be edgy pre-teens or prominent math professors, it's really just a self-association thing. There are informal self-organized meetup groups in various cities, but not much different than groups who meet to discuss poetry or literature or video game speed-running. Some ideas have bubbled around more than others, like any memetic spread, but I don't think it's accurate to say that the people who associate there all agree on things, just like HN does not either. And nor is it any more of a clique than HN, or Twitter for that matter, even though you'll find some people on Twitter are more prominent and more vocal than others.