mapping user name to group name

Hey guys... first time caller, long time listener.

Don't ask me why, but I would like a quick way to map a user name to the
group name that the user is part of. Using pg_user and pg_group makes
this a little tricky. You can obtain usesysid from pg_user, and then get
the groname from pg_group based on it. However each group has its own
usesysids within an array of integers. I have no clue how to search
within an array using a select statement.

Am I making it harder than it should be? Is there a sys table already
out there that is doing this mapping for me? How do I do this (with
minimal hastle)

thanks much
matthew