They unofficially team up, often helping each other with tames and materials. This will be such a bigger issue if they cluster the small tribe servers since people will just build up so big that no one else has a chance to build. The reason that people enjoy this game mode is that of the competitiveness trying to build up with 5 other players to be the "alpha" of a server, then once you have reached that goal other servers will come crossark raid you.