To enable wheel group, uncomment the following line in /etc/pam.d/su file.

# Uncomment the following line to require a user to be in the "wheel" group.
auth            required        pam_wheel.so use_uid

Once this is done, add the user to wheel group using useradd or usermod or gpasswd commands.....

Now the users which is added to wheel group alone can access root....!!!