Quests and Missions can be realised by players groups (and at long term by non-players groups). It's recommended to use at the maximum this ability to create a true social interaction between players.
A players group can work on a Quest or a Mission together (all the players work on the same Mission, eventually each one on one special objective if multiple objectives) or in cooperation (if several Missions should be realised, the players should act in parallel to accelerate the Quest).
Moreover, it's also recommended to use at the max the oppositions between players or between groups ('bodyguard' Mission against 'murder' Mission (attack/ defence) or 'find the treasure' Missions in parallel (competition), for example).