switching between groups

[Adil Katchi]

I was just wondering if anyone has any ideas how it's possible for a user
that belongs to multiple groups to somehow limit his or her own capabilities
by using only one of the n groups that they belong to and be able to switch
between these groups?  For example, if userA belongs to groupA, groupB and
groupC, can userA enter a mode that would force it to only belong to groupA
(or groupB, or groupC)?  UserA whould be able to switch between these groups
and back to normal (ie. belong to all groups).

Any help would be appreciated.

Adil