How do I list the groups that a UNIX user is a member of?

Problem

With I can list the users in a group. How can I do the opposite - that is, list all the groups that a user is a member of? (I need this to find the name of a group that I know a user who is a member of, but I don't know the group name...) Clarification: This question is complicated by the fact that I am not the system administrator, and that I don't have root permissions (far from it). Instead, I am part of a group of "moderators" that administrate a small part of the system - to be specific, the physics branch of the student union at my university's central IT system. Within our branch, we have a bunch of different access groups for people who are in charge of things. In this case, one of two people responsible for something (it doesn't matter what) have been replaced, and I was looking to examine the other user to find out what access rights I should give the replacement. I have now been able to solve the immediate problem (the new guy not being able to access a folder) by examining…