I can join the same game (my own) from my own computer 15 times so I do not think that is the issue. I'm sure it's just an IPX issue. One thing I can suggest is just hosting a normal NovaWorld server (your your external ports blocked) and have your party use the tool linked below to enter your LAN IP to create a startup.htm that will auto join your local server.
http://novahq.net/files.php?ID=18