I'm not quite clear what your problem is, but anyway....
When you create a user, he is assigned a username (login name) and a numeric uid. He is also assigned a PRIMARY numeric gid (which will also have a group name).
The username, uid, gid (primary) are to be found in /etc/passwd.
You may also add the user into 1 or more SECONADRY groups; these are specified in the /etc/group file.
Note the RHEL and derived systems default to setting the default (primary) gid to the same value as the uid, in order that each user is in his own group by default, thus preventing security leaks via group ownerships.
Contrast with trad *nix ( and other Linux distros) that have the same default group for all 'normal' users.
See this doc http://www.linuxtopia.org/online_boo...ion/index.html