A friend and I have been trying to complete each mission type on damnation. We quickly noticed that most of our games will never fill so we tried some experiments.
I would start a match alone on damnation and have my friend attempt to join it in progress. Most of the time it would just put him in a new lobby and completely ignore my in progress game. We found a way around this by queueing up and then canceling if it doesn’t start joining a game within about five seconds. It usually takes a handful of tries but it’s a guaranteed way to end up in a match that’s already running.