When assigning a user to a group of which it's already a member, C5 throws up at MySQL error (see attachment). Minor, obviously, but an error message such as "The user is already a member of this group" would be a nicer event handler.
I guess the 'Add Group' function should exclude user's current groups, even with search, or at least shouldn't allow choosing the same group again.
Tested with r1522, same issue.
x1 
okhayat
Group assigning problem
Tested with r1522, same issue.