Still seeing this request quite frequently in my website logs, I thought I'd expand the topic a bit more, given the fact that I've somehow left out the most obvious way to confirm the group membership for a Unix user.
Group membership in Unix
Every Unix-like OS is bound to have a group command, which is aimed to help you confirm Unix group membership for any user known to your system.
The easiest it to run it and specify a username in command line. The result is a list of Unix groups:
ubuntu$ groups greys greys : admin www-data
If you're looking at confirming the membership for a few users, you can specify usernames in the same command line:
ubuntu$ groups greys root greys : admin www-data root : root
There you go – hopefully this satisfies your interest. Good luck with Unix experiments!