Hahaha glad that you have friends! It should be as simple as plugging in more controllers and pressing any button when you've loaded a map (only one person can play on keyboard)
Ah. I plugged in a keyboard and a controller, but that did not seem to work. Now I plugged in two controllers and it seems to work. I think having two different buttons for weapon pickup and release is a bit confusing (maybe I am just not playing enough controller games).