Multiplayer on PC is plagued by client disconnects. This includes two players in the same city and on the same ISP’s network.
We have only played 2 sessions. My 2 friends join my game (I’m on a pretty high-end computer with Gigabit bandwidth to Cox Cable). The longest we have gone without a disconnect is probably 10 minutes…
Is this a Steam issue, or a Generation Zero issue?
I play on the Microsoft Store version and never have this problem, but I would start by posting the specs of your PC and the specs of your friends PCs. Then, maybe repair your game files through Steam. And possibly uninstall and reinstall Generation Zero.
All of you should probably do this. I know it can take a while, but who cares if it fixes your problem. 
But, maybe it is someone’s Wifi that is causing the problem. Or maybe someone’s PC needs a restart or something. Just start trying things and eliminating possibilities. Maybe try and hardwire your computer to your router. That is how I prefer to play.
Hope this helps.
ProCaptainAJ