What level your groupmates are has absolutely no bearing on the quest you are doing. Your own level *may* affect the experience at the end when you hand it in, but questing for the most part is unaffected.
Mob kill exp is reduced noticeably with a higher level character. When I was leveling my hunter, at about level 20, a guildmate brought his main along that was over level 50, and I was getting less than a quarter of the exp per kill, rather than half. Within some reasonable limitations though, I don't see this affecting exp in-flow very much. Don't group with someone otherwise way out of your league, and you'll be fine.
Only the exceptions can be exceptional.