A brief google leads to this node, which deals with listing a user's secondary groups, you can adapt the answers from there to suit your needs. The primary group of a user can be retrieved with getpwent. You may also want to use User::pwent and User::grent for a nicer interface to those functions.

In reply to Re^3: count of USER UNIX groups by Anonymous Monk
in thread count of USER UNIX groups by Apronline

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