I think this is a basic question and I am missing something quite simple.

What is the procedure to setup a system so that two users can su to root.
I attempted adding those users to wheel group (/etc/group) and experienced
the following problem,
The user logs in using password xyz, he then attempts to su to root by
typing 'su'. He is prompted for the password (this means he is in the
proper group?) he enters the root password 'zyx' (no thats not my root
password!) su replies "Sorry" and exits. The user then logs onto the
console as root using the _same_ password. 
Another odd behavior:
If I log on to the system and then type 'login' I am prompted for a
username and a password. If I enter username I just logged in as, and that
user's password the login fails every time.  What is going on here?
-Thanks