sorry I can't figure it out from the documentation :P
That's only possible when you logon as that user and go to his Personal Preferences (2nd tab)
As a system admin you can't see to what roles/groups a user belongs.
You can use a 3rd party tool to do this.. E.g. "Impact Suite" from a dutch company. Google for it...
If you have access to that user, have him log on and then go to 'My Preferences' and then to 'Personal' tab.
It'll list all the groups and roles there for that user along with all other information like namespace, capabilities, email etc.
thanks everyone ;)