As Humans we are programmed to seek our place in the world, to question ourselves and to never be happy with what we have. So that's why groups be them religions, political, school fraternities or people of similar ideals like Neo Nazis come together because it makes them feel apart of something and more whole than if they weren't in that group. This is partly because Human instinct is towards running in a pack rather than alone, but ideologically it makes us feel less alone.
In my life I have experienced many such groups and is occurred to me while watching Accepted that exclusive groups where membership is filtered to only allow the "right sort of people" in normally feel hollow because when you join such a group you are buying into the group founder's vision and not something you can influence yourself.
Inclusive groups are groups of people who all try to get along. And since everybody in them puts effort into getting along, being part of the group feels more whole as everybody has a viewpoint that is heard. A lot of inclusive groups like sports clubs, music and other groups are so inclusive that they transcend any physical place and become an idealogical movement like Wikipedia (the open source encyclopedia), The Skeptics Movement (critical thought in search of exposing truth), Parkour (be strong to be useful) or any number of civil rights movements.
However the biggest inclusive and exclusive groups in the world is society itself. In some parts of the world people actively segregate themselves in order to find their place in the world and in other places people pay for services like national health so that everybody is more equal.
So what I am trying to say is that there are a lot of people in the world that try to find their place in the world by excluding other people or ideas. Even to the point where they suppress freespeech and commit hate crimes against people who they consider to be apart of another group, this is probably a natural instinct for Humans, but I think if we all tried to be a little bit more inclusive by helping others, listening to others and giving others our time then the group that we are already in, society, becomes a much better place.