Two suggestions.
1. Would it be possible to scale characters to the area they are in (like GW2)? That way you'd get xp there, could have fun questing with lower level friends and much more.
2. Let groups share quests already completed. Lets say you're in a group with a lower level character. You have already completed the quest he's on. But he can still share it with you, although you won't earn specific loot and rewards from it. You will not experience the dialogue again, but will be able to see the NPCs, enter areas and interact.