When you Play campain in Coop, there are lots of missions like “Go to Zone X”.
When you enter those zones in a team - only the first one entering get the quest ticked as done.
Then all party members need to leave again, and the next can “fist go to zone” so that he also get the quest done. For that, all other party members need to leave the targetzone bevorehand.
Assumption is - only when a zone is created for a party, the first geht the quest done. So you need to destroy the zone (all leaving) and recreate it with other player. And this for each zone and every member. This are a lot of very slow and unneccessary zone changes, making your server problems worse, because you have multiple times the zone transitions as normally needed for the quests.