In my attempts to harden a Redhat ES 3 box I ran the following commands from a Linux security book.

/bin/chgrp wheel /bin/su
/bin/chmod 4750 /bin/su

This was after I added one username to the wheel group under /etc/group Now no one can su. I'm doing all of my work at work instead of remotely trying to fix this problem. I get a "incorrect password" error for any username trying to su either to root or another user. I know the passwords are correct, I even changed one to be sure.
I changed the ownership of su back to the root group and ran chmod 0755 on the file itself. What am I missing here? ANY help/advice is appreciated.

chmod 04755 fixed it. Now how can finish what I was attempting? Added the username right behind root in the wheel entry in /etc/group I was certain I'd done things right.

Did you logout and re-login the user after you had added him to the wheel group? Group membership is set at login time, so changing /etc/group while a user is logged in will have no effect on their group membership. If you don't want to logout and re-login, you can also use newgrp to become a member of a group that you've been added to.

you want only members of group wheel to be able to su to root?
i've done this with pam once but i don't remember exactly how
you have to put something like this in /etc/pam.d/su: Auth required /lib/security/pam_wheel.so group=wheel
do a search in google for examples