Host hosts a game ingame and opens ports 27015 and 1000.

Clients add this line into their DamageInput.ini in /DamageGame/Configs RangedBindings=(Name="P",Command="Open HostIPHere") Put that as the last line in the "RangedBindings" section.

To join, go to the HQ and press P.

You can also put "OPEN IPHERE" after the game shortcut target, but that way you won't get your saves on the server and will play as level 1. (The game loads your save after you go into the menu. By doing this you skip the menu and go straight to the multiplayer lobby)

Works with any version AFAIK