@puddel90, why do you think someone would ask first if you were the host or not? because that matters. you cannot join some random server with this on and expect it to work

Just played a co-op session a few hours ago. It seemed like the plug-in wasn't working. Is there something I'm missing here?
Before you ask, I was not hosting it.

Thank you Jedediah. Coming in clutch for real

For co-op campaign, setup a network game and choose the game type "Cooperative Play". Then others can join you similar to multiplayer.

Does everyone playing need this or just host?

I wasn't aware marathon hap cooperative campaign play. How is it activated? I want to play with one of my friends :)