Probly cause, in anything with an open socializing system, there are people who go around trying to get on as many peoples friends list as possible for the sheer hell of it.  I'm guessing there are similar people for groups.

I have a 3 (sometimes 4) other friends who are looking for somebody who won't spastically molotov right at the end.  My ID is Stoglette, could someone invite me to the Steam group?