a level 70 killing stuff for you, grouped or ungrouped, will yield very little xp to you. They have tweaked the formulas on and off the last few patches, so I'd just go try it out yourself to see instead of depending on websites that may be inaccurate. The only reason to use a 70 to kill stuff is to do quests faster, which is the big XP payoff.

a 70 can heal you however if you do all the damage to the mob, and if the 70 is not grouped with you you will still get full xp. if the 70 groups with you, or does some damage to the mob, you will get very diminished xp.

  • regular mob grind, you should use all characters around same level, can't pull too much. a 70 can follow you around and heal you
  • instance grind, you can use a 70 and even though the xp gets nerfed you are depending on killing stuff fast enough, fast respawns (instance reset), plus the extra xp of elites.
  • quest grind, the 70 helps your lowbies get quests done by zapping mobs that drop quest items or are quest goals.