One common approach is to create a group called "wheel". Then:
chgrp wheel /usr/bin/su
chmod 4550 /usr/bin/su
Now su can be run by root and by members of the group wheel. So you put some users in that group to allow them access to the su command.

It is traditional to call the group "wheel", but I don't know where that name came from. Possibly from "big wheel" which is slang for important person?