My post is related to personal experience with in-groups as opposed to research on the subject. There is already a thoughtful post on the nature of these kinds of groups and their impact so I'll try to go in a different direction to add to the discussion.
In 1989, I was the first and only person in my rural high school of 600 students to be openly gay. I don't mean 600 students in my class, as most urban kids are used to. I mean 600 children from grades 7-12 in one building. The small class sizes made for some seriously tight in-groups and being ostracized for being different had a bigger impact because of that. There were no gay student groups and no discussion about it at all except the daily ridicule, scorn and violence that I received from my fellow students and to some degree even the teachers.
Needless to say, I did not fare well trying to be part of the in-groups of my school. I was my own little out-group and in respect to my orientation the only person like me that virtually everyone in my school had ever encountered. Even the out-groups became impenetrable cliques to me for the most part. Stoners, band geeks, and even the truly unpopular kids forced in to the back corner at lunch wanted very little to do with me because of the stigma placed on me by more popular kids in the school. I did my best to go my own way and get on with my day, just waiting to go home and get away from all the in-groups I couldn't get in to.
What I found after a while though was that there were other kids who kind of fell through the cracks of these groups. For example, there were 4 kids in my school who were skaters, and one or two who identified as goth. They really didn't have a lot of people in their out-group and often they were all different ages so didn't sit together in class or get much opportunity to band together against larger, more popular groups. So what ended up happening was that we started to talk to each other based on our mutual feeling of oppression and exclusion. Suddenly, kids who didn't really have anything in common with me at all were watching out for me in the hallways and defending me against bullies or I would find myself helping other kids work through their sense of alienation or stepping in to prevent them from being abused at home. Our only real sense of identification with each other was actually because we were all treated the same by the larger groups. That is to say, they hated us for different reasons but they treated us all equally badly because of it.
Years later, I would realize how much that really shaped me in my willingness to open up to people and how much this strengthened my ability to identify with people who were different from me. Conversely, I saw through facebook and acquaintances who kept in touch with the various in-groups and cliques of our old school what really happened to those popular kids. For the most part, they were so comfortable and complacent being at the top that they were unable to adjust to life after high school which demands that you be open and adaptable. A large majority of them never left the comfort of our small home town- despite the fact that jobs are less and less available and the quality of life is severely limited by its isolation. They live small, unimportant lives with no real chance of advancement, while my in-group of outcasts have all (every single one!) gone on to wide-open lives and amazing opportunities that were only made available to us because we learned to respect other people's differences the hard way. The world works on diversity and has increasingly come to value that. I can only thank god that I wasn't one of the popular kids in school. I might have ended up stuck in a dying town with no real prospects for a decent life. Sometimes you're so out, you're in.