well when one of my toons fell behind to far i would put them out of group and have them tag a mob then the other 4 group members kill it. He would receive 100% exp. But trying this with the 70 having the lowbie tag first then the 70 killing in one shot would give like 10% exp. Now if you lowbie does like 25% + exp your exp will go up. But all in all its not worth it unless you are aoeing.