Private Server Error #2048

Ok I just opened up a Private Server to my friends. I have all the Ports set to redirect to my home computer just fine. Everything is set up correctly, but when they open and sign in as admin they get an error #2048 when they try to join, even if I myself am not in the server. I don't know how or if I need to give each person specific access or how to allow all users to join (if that is what needs to be done). Any help is wonderful. I don't know how to get them in.

Also I don't know how to allow people to register accounts on my private server. I am only allowed to log in as admin and obviously if multiple people are playing then I will need multiple accounts. Somebody Please Help!

Who's build of the server are you using?
Trapped's?