A tribe consists of both members and followers.
Members share their tribemates content, and their content is shared by their tribemates. In order to become a member you must either receive an invite from the tribe's Chief, or the Chief must promote you from follower to member.
Followers share their tribemates content, but their content is not seen or shared by their tribemates. Followers may join any tribe, unless that tribe is protected. They may also participate in the tribe council.