Do you mean that you want the player to leave and join groups at anytime?
Well, to leave a group, you use
[player] join grpNull
So, to join a group again afterwards you should be able to just:
[player] join _leader
_leader is a local variable which is the group leader of the group you want to join.
Make sure _leader is defined as:
_leader = (leader groupname)
I'm not near a computer with CWA on right now, otherwise I would check. Hopefully this'll work
Regards,
Gruntage