I've played with randoms before searching a standard Insane with one friend. We were on Impact. We managed to beat it. I didn't invite them to a party or anything but they didn't seem to know their roles. For example, whenever someone else picked up stuff to deposit, I'd message them to say "hey, I'm scout. That's my job". I would just melee them if I wanted them to get off their turret to upgrade it and stuff. No one left. Was that just a coincidence? Maybe because it was insane, they had more of an incentive to stay? This was before the bounty upgrade btw.
I guess on casual, people would just go for bounties but harder difficulties means people might actually stay. (unless they notice you're going nowhere; keep failing)