Can I use Selfregistration with existing users?

[Kailoi]

Something I thought to be incredibly easy turned out to be probably impossible...

What I tried to do:
I use the Selfregistration/FrontendUsers/CustomContent-Module-Combination to show different contents to users, who are members of a special group.
I wanted users of group1 to be able to register themselves to a new group2 at a special page. I wanted to use this to create a kind of game and the changing of groups (From 1 to 2 to 3, etc) is comparable with reaching the next level.

The Problem:
As I see it, Selfregistration seems to be for first-time-registration only. I couldn´t find out, how to make it work for already existing users to increase their memberships to other groups.

My Question:
- Ist there a way for user to register themselves to more than one group?
- If yes, which command lines and configurations of the modules are needed?

[calguy1000]

sorry for the late reply, but no, there is currently no way for users to register themselves for different groups, etc. 
this was designed for first time registration only, as you mentioned.