Hi,
I have root access on Linux red hat server.
How do I see the current user list and group they belong?

ThankS!

current list? Do you mean those who are currently logged in? try
who

If you mean the list of users, look at /etc/group

Sorry.
I mean the user name list and user group name list that have been set up the admin.
I just have root access but not a admin.
I want to change one of the user's password and I also want to see who have been set up as a user on this machine.

Well, as long as you're not using NIS or anything like that, you could just do
cut -d':' -f1 /etc/passwd
to get the list of users, and then look at /etc/group to see group membership. As far as I recall on Redhat (I use SUSE myself), each user will also have a group associated with the account (with the same name as the username) - and this will be their primary group. /etc/group will list any "secondary" group memberships.

Cheers
ZB