I am in several regular groups and one secret group. The regular groups I am in - one is a local group, one is a memorial group for someone who died young, one was started by a friend for his hobby.

The secret group has a lock on its tab and says "Secret Group - Only members see the group, who's in it, and what members post". No one who I am friends with knows I'm in the group aside from people in the group. Not sure what more to say - it has a wall which is somewhat active, although content was better and more diverse when the group started, now it is mostly 1-2 boring friend-of-friend chatterboxes. It also had a chat room, although Facebook killed group chat rooms a few months ago.