gpasswd -a firstuser video
gpasswd -a firstuser mysql
gpasswd -a firstuser games
gpasswd -d firstuser mail
(sorry if I repeat, it's written right in "man gpasswd")
if you want to remove a group from /etc/groups use groupdel
if you want you can edit also directly the group file with "vigr": if you don't like vi (shame on you
) set a different EDITOR environment variable before launching, like "export EDITOR=pico",for example.
remember that you have to logout and relogin with your user for changes to take effect for him.