I have read a lot of threads concerning players leaving dungeons/Duty Finder do to "new players" not knowing their role. Here's my suggestion:

Have the system pop a reward for completion if a new player (hasn't cleared yet) is in the party. Make the reward something big. They already do this with the roulette system providing an increase in rewards for Tanks and Healers that is... I feel this would make players more eager to help new players through whatever dungeon they're in.

just my two cents...