As of November 11, 2012, the PSUPedia and its affiliated wikis have made the transition to read-only mode. While edits can no longer be made, we hope you will continue to enjoy the data collected by the community for years to come!
From The PSUPedia
Party missions are unique missions that require at least two players to initiate and complete. As with story missions, there are a few special conditions that party missions follow. First, a party must be formed before the mission can begin, however no NPCs may be in the group. Second, Scape Dolls cannot be used; incapacitation leads to immediate failure. Likewise, if a player leaves the party for any reason or if the leader is changed, the mission will end.
A future update will introduce a time attack ranking system called GUARDIANS Limit Attack which will focus on the S2 difficulties of these missions.