user management questions

["Blackburn, Marvin" <Marvin Blackburn glenraven com>]

I am new to the shadow password file processing and Have a question.
How do I properly change a users primary group.

Can I just enter it into the /etc/passwd file?
How do the changes get propogated to /etc/shadow and /etc/gshadow?

Any help would be appreciated.
I am running redhat as 2.1

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Marvin Blackburn
Systems Administrator
Glen Raven
"He's no failure.  He's not dead yet" --William Lloyd George