Public (Anyone can join)
Public Approval (Anyone can apply to join, but must be approved)
Private (Hidden and invite-only)
I chose public approval. but i've yet to ever see an offer for me to approve somebody... it just lets them join as far as i can tell. until that bug is squashed i'd like to make it private temporarily to stop adds but i don't see any option to make it private after the group is created either...