Just so long as the party leader has met the requirements to enter the battlefield, let whoever wants to help do so, even if that person hasn't done any of the mission or quest line. I know the 3 mini-expansions from 2009 are modeled this way. Let's expand that! It can be so disheartening to want to do missions and certain quests, but not be able to find the bodies needed to enter the fight. Even when there are good-natured people willing to help who can't. We're lucky that the old missions can be done in smaller groups, but I am especially fearful with the upcoming Seekers of Adoulin expansion that the missions will be designed to be challenging to level 99 players, and that people will have difficulty finding groups for them.