How do I list the groups that a UNIX user is a member of?

You can also use the groups command:

[root@ftp ~]# groups root
root : root bin daemon sys adm disk wheel

if all else fails there is also good old grep:

[root@ftp ~]# grep root /etc/group
root:x:0:root
bin:x:1:root,bin,daemon
daemon:x:2:root,bin,daemon
sys:x:3:root,bin,adm
adm:x:4:root,adm,daemon
disk:x:6:root
wheel:x:10:root,admin

$ id [username]

Found it - way later, but I did! =)

Just as

$>pts m [group name]

lists all the members in a group,

$>pts m [user name]

lists all the groups a user is member of. It was too simple :P