Each quest should have a modifier for the amount of experience based the number of levels the group has rather than based on the highest level person in the party. Thus if the group is made of only two players the group would get more experience for completely a quest, whereas, if the party has 6 high level players then the experience would drop. This will reduce the penalty for having just one high level player in the group, and give bonus experience for groups that run quests with a lower number of people.
Example: if the quest is a level 10, then the 0 modifier would be based on 60 levels of experience. If a party of 6 x 10 level players play the modifier would be 0, but if only 3 x 10 level players play they would get more experience.