Everywhere you go there's going to be cliques. While I wouldn't say there are people on TCS that purposely leave others out, there are quite a few people who are closer and talk off-site as well. You can't really blame them for having close friends on here.
As for feeling like an outsider, let me join the club (oxymoron, no?)! I'm most likely one of the youngest on the site, and I can't really jump into discussions about work and kids, etc. It's a little harder to get to know others better on here because I can't PM, reply in the IMO forum (I wish I could!
) or become a premier member yet.
If you feel left out, try jumping in to more conversations. Even adding a few words to a thread can help us to know you better and vice versa.