Oh wow I just found this while trying to find an invasion solution that would work with Death and Decay. I tried it with Invasion Simulator but for some reason the wave progress has a tendency to get stuck thinking that extra players are monsters or something, forcing players to either spectate or kill themselves until only one remains. Here's hoping this doesn't happen here.
So after trying it, it works like a charm.. only issue being no real way to configure the waves beyond wave count
Perhaps allow players to set things such as spawn delay (time between enemies spawning), enemy count multiplier, etc.