This "not fitting in" stuff comes up from time to time. The folks who have been the most cliquey and exclusive know who they are and they'll knock themselves out welcoming everyone to their little group for about a week or two (they don't want to appear cliquey and exclusive) and then it goes back to business as usual with the thread hijackings, inside jokes, in-banter and ignoring anyone who isn't in their little club.
I'm not anti-clique. I'm pro-clique. I see nothing wrong with it. People make friends on the internet, but they aren't friends with every member of the board. How can they be?
Eventually, the clique either implodes with members fighting with each other or they just get sick of each other...and a new clique starts up. Very often the new clique is started by someone saying, "I'm left out" and all the other left-outers join them. Then they're the cool group until it all comes full circle.
If you aren't part of the clique, don't feel bad. Start your own. Seriously. Just start a thread titled, "I'm not part of the club" or "I'm not popular here" or whatever and then ask if there is anyone else like that...soon, you'll have your own little band of merry DISers.
Just as in high school, the cliques aren't all they're cracked up to be. And just like in high school, most of the people there aren't in the in-club. And many of them don't want to be - find those folks, they're everywhere!