In order to run local coop you need to complete the prologue (Training at the start)

1.       Install Radmin VPN

2.       Create your Network by pushing a button "Create Network". No registration needed. Or you can use Public networks, disregard next step if so

3.       Get your friends to join your network. To do it they need to know Name of the network and password

Here is also a guide on how to create and manage the network.

To host a server PC you need to make a lobby.

Here is how you do it:

1. Main menu -> Play -> Play Campaign ->
   
2. In Online Options make sure that Game Type set to LAN
   

To join the created game:

• Make sure that your game versions are the same
  
• Make sure you’re in the same Radmin VPN network
  
• Launch the game, then press escape and choose “Matchmaking” -> Find games
  

If it doesn't work -  try the tricks at the beginning described in this guide first five steps.

And here you go! Playing with your pals!