The names of the groups to which a user belongs can be seen in the Member Of tab of the user properties window (The user can also be added to new groups or removed from the ones he belongs to by using this tab).
The Values (or group names) are stored in a multivalued LDAP attribute called memberOf.
Note: When required the primary group of a user can also be changed using this tab. The name of the primary group does not appear in the memberOf attribute. The SID of the primary group is displayed separately in an attribute called primaryGroupID.