I have encounter this problem when I was trying to help my friend who want to learn iOS programming but at the moment cant afford to buy a Mac. I have Macbook and Mac Mini. I was avid use of linux in my past and I have used XSessions on linux where I was exploiting multiuser service from a linux machine and had my personal GUI or XSession. As Mac is basically unix based system. I am wondering if it is possible that I can setup my mac mini such a way that my friend can connect to his personal xSession over the internet to my mac mini and start coding on my mac mini.
This is not possible, since the OSX GUI doesn't run on X Server. Furthermore, there is no support for simultaneous desktop logins. You could theoretically run an X server on OSX, but you'd be able to run only apps that use XQuartz, not native OSX apps.
You could allow him to access your Mac via Screen sharing or VNC, but it wouldn't support multiple users.