After installing and loading the mod, start up your own server (in "Multiplayer"). If all your clients (devices, or windows that the game is open in) are on the same network this is unneeded.
Now you're in the game as node 00 because you're the host (this is important).
Now, launch the game again so that you have it running twice at the same time. You can also run it on another device with the game.
In this second instance, join your server. If you're running both instances on the same computer, you can go to "Specify IP Address" and type in "localhost" (without the quotes of course). This should boot you into the server you set up earlier as node 01.
Now set up your characters like normal, and on the second instance, open the console (try the ` key) and type in "setbot 00". Now the second player is following the first and you're done!
If you ever need more team members, just open up more instances of the game.
If any of this confused you, Please let me know!