Ability to flag users, or add to a second group

We recognize our donors - people who support the running and upkeep and expenses with free-will donations. To do so currently, I had to create a special non-ladder group and when someone donates, I move them into that group so that a special donor recognition icon displays instead of the stars.
The down-side of this is that now they are not in the ladder groups, so dealing with rights that one may gain with more posts, or other configuration for the user is impossible
Would like either ability for multiple groups - OR, a special "status" check mark we could put in their member profile that would change the icon or other status somehow.