In the Perl debugger, the "m" command lists the available methods, including following @ISA and showing you XS routines that are not apparent in the *.pm source code.

Update: For your specific question, it appears that the group() method is what you want. However, a user can belong to more than one group. If you can't deal with more than one group, then you might want to just deal with the first group name returned:

my $group= ( $db->group( -user => $user ) )[0];
# or
my( $group )= $db->group( -user => $user );
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        - tye (but my friends call me "Tye")