Allow users to change usergroups
 

Hi* a search didn't turn up anything so I'm posting this in the hope that someone knows the answer.
I need to be able to give users the ability to join a usergroup (and leave it when they like)
The reason for this is that we are developing a mystery picture game on our site and need to hide the images from all members but the mystery game group. Those in the group like to play it for some months and then leave it for a while before haveing another go at the game.... during that time they would probably like to unsuscribe from the game and see the site like regular members.
I'm aware that I can join them up and remove them but to be fair with 1500 members that's likely to be a full time job.
What I had in mind was a subscribe button in order for them to join the new (additional) group.
As I'm new to VB (old smf user) I'm a little lost.
Regards*
Steff