If the user/groups you are talking about are postgres users and groups,
this is it:
select * from pg_catalog.pg_group where (select usesysid from
pg_catalog.pg_user where usename = 'user') = any(grolist)
The place to find this kind of thing is the Postgres Internals section
(system catalogs) that desribes the system tables. "any" is an array
function which is needed as the users that belong to a group are an array
of INT user ids.
Hope that helps,
Bas.