I've tried to join a public server from a little lanparty but it seemed as only one person could join a server, as soon as th second person tries both got kicked.
Someone suggest me to use a different net_port for every client, but this seems a bit to complicated for a lager group of people to change this settings manually. So my suggestion is to add a option in the settings-menu to use a random port instead or check if the port is not allready in use (what shouldnt be that hard).