For example, I have a forum with an international audience, and to ensure user mistakes (posting in an inappropriate sub-forum, for example) or spammers, or inappropriate posts get promptly handled I have created a "Moderators" group. When a group is created an additional column appears on the Permissions screen. I give those added to this group additional permissions that normally only administrators get - but they don't go as far as allowing them to delete the forum!
In another: http://www.seahawk17.plus.com/forum.htm I have created special groups for:
"Sellers": users who want to advertise their boats but don't have any other interest in SeaHawks. There may only post and reply in the "For Sale" area.
"MyBoat": for users that request a special area in which they can create their own sub-forums. (Actually, as this area grows I'll probably need to create a "group" for each individual boat.)
In another, for residents of the area where I live, I have created a group for "Residents", It seemed a more appropriate name than the default "Members", which goes unused. Another group, "Shareholders", allows access to the otherwise invisible sub-forum, for those who are part of the management company that looks after the communal areas. Again I could have used the default group "Members" but chose not to as it helps give the forum a more customised feel.
Just a Volunteer Nabble Helper - because the nice folk at Nabble have helped me! GregHelp - Building a set of answers to Nabble FAQs.