[yason]

Is there a particular reason you'd like to join with one, some or all of the cliques?

It's quite natural for people who get along to group together because in the group you can act and speak more freely with the friends you know. When a new person joins the discussion easily dies because everybody suddenly has to start thinking again what you can and cannot say. It doesn't mean the people in a clique would be actively rejecting you but just that they're quite happy as they are and more interested in relaxing on their lunch break rather than working to make new friends.