Listen server problem

[mollyrulz9999]

Hey guys!

Me and some mates have been trying to set up a listen server to play online, however we are not able to start one up. When a friend sends me an invite to join the game, it says 'Server is not responding', and the same thing happens when I send them an invite. Nothing appears in the friends list either about a new server being created. How can this be fixed so I can play with friends?

cheers,
mollyrulz9999