Chose to have people join groups by Moderated or Unmoderated process

Groups can be made visible to logged-in users and have users request to join.

 If a group is set to allow auto-join, then new users will automatically be added to the group when they as to join.  

If the group admin sets it for moderated, then an email is sent to group admins when a user requests to join a registered user group to review the request.  See sample email.