Have you made the necessary changes to /etc/passwd, /etc/group, and /etc/nsswitch.conf?
At the end of /etc/passwd add a line at the end that looks like this:
At the end of /etc/group add a line that looks like this:
In /etc/nsswitch.conf (it may be different on RH) make sure the lines for passwd, shadow, group, and hosts has "nis" in them.
Hope that helps.