Ok i'm not really sure if pc's far under your lvl get xp at all when grouped with you,but for now i assume they will
the guy in the other post is right (with the mage)
just hook(follow) all lower lvl's on to your mage (20 or more lvl's higher than the mobs)
agro as much as you possibly can (without dying) and spam AE untill they die
perhaps the players get less xp but if they get any at all this might be a pretty fast way to lvl
for horde:go to fenris isle in the sepulcher.the gnolls in the castle are perfect for this,they also have a very fast respawn