There is an host (he plays too) and players.
for the host:
From the main menu : solo => campaign => host (at the bottom)
Configurating the host :
Name : "it will appear on the server list"
port : "let it"
max players : "no draw ^^"
password : "no draw"
Your server is created. Now, the players have to connect on.
for the players:
From the main menu : Multiplayer
They have to search a server named as you have done
If fact, the game just create a server, like so many servers, but played on the campaign. That's the only difference.