Aha. I think I found what I am looking for. Rather than giving each group leader admin permissions, I can set the group to "Request" so that other users can ask to be added to the group. Perhaps group leaders can send the url to the group via email to any friends they want to have join the group for access to the forums.
A couple more questions, is it possible to list all of the groups with group avatars so that guests can view the list?
Is it possible to display group avatars and/or avatars in the member list?