I believe that most of the groups are formed due to the presence of a common factor. I am now part of a number of groups because I have something common in me. Another reason for being a member of the group is due to the sharing of similar test. It is a very common reason for being a part of a group. It is possible that I and rest of the group members are sharing a same preference; it will make the group members think that the individual will be suitable for the group. It actually works in most of the cases.