Adding users is done by the useradd command
eg: useradd -m -G users,wheel,audio -s /bin/bash username
the -m switch creates the users home directory
the -G switch adds the users to the specified groups. Note these groups must exist in /etc/group

To administer existing users, the gpasswd command can be used:
for x in cdrom cdrw usb ; do gpasswd -a username $x ; done

This just adds existing users to new groups that were not necessarily specified when the user was first created.

Note, to create a group use groupadd
to delete a group, use groupdel