How to setup "Accounts for other people" that they can log in at there home

I’m not sure if I did right. I have add an user/ college to my Server Manager. That was easy and selfexplainig. But how the user now can log into the my server.
Which port I need to forward? or How can my college now find the Server Manager on the Browser. Or how my college can now log in from a different place? Is there any detailed step by step description how to do it, would be much apprechiate and helpfull.

The easy way: use UPnP and a DynDNS service.

Thanks for the advice! It works perfectly!