It should be possible to invite more than one player for a co-op quest, letting only 2 of them to make the quest, and the rest of them to watch it like it happens in pvp. It would be a very good way of teaching the new players on how to make the legendary quests... If you pay attention, we have always new players coming (every week), but only a few stay. I think this might be one of the reasons why they leave...
