Your problem may or may not have a simple solution. Are your friends using you ip address to connect? Or is the problem that it doesn't show up in the server list? Are you behind a firewall/router? If so, have you forwarded all the ncessary ports? Are you running the dedicated server binary or the mp game to host the server? If the former, what's your command line and config look like? If the later, are you setting it up as dedicated in the setup screen?
"Do you expect me to talk?"
"No, Mr. Bond, I expect you to DIE."