OK so here's what I understand:
 * Current join_group is only used for the logged in user to join a group
 * Curator should never be in place where he needs to join his own group

Given those to clean up some code and keep it as minimal as possible I suggest that we:
 * `profile_to_add = request.user.userprofile`
  ** This is also remove the security problem, of having people subscribing others to groups!
 * `is_curator` is not required
 * `is_superuser` is not required either and although it doesn't hurt I suggest that we don't have it there. I will help superusers get closer to user experience and resolve problems. E.g. people complaining about not being able to join group, group is invite only, admin sees the "join" button doesn't understand it's invite only, unless he goes through the admin panel. Superuser's can go fancy and do whatever through the admin panel so we don't limit functionality.

Given we do these, we don't need the new tests either!