I did this last night with my warlocks felguard and here is what you need to do:

-You need ATLEAST 2 lowbies or for some reason it doesnt work. I boosted 1 shaman the other one died i just had him on follow on my main shaman still got the exp but it was splitted.

-Do atleast 1 damage with one of your slaves to the target then kill it with the Demon/pet.

PRO tip if your only leveling one:

Group 2 lowbies and once you reach a spot you want to be for a while kill the one who is not in need to be leveled. Place him ATLEAST yards away but never more then something like 300 yards away either.

Now for some reason the XP will not split between the 2 (I assume its because hes not near enough to get the kills) But if he gets to far away the xp will go back as you werent grouped.

I boosted 1 shaman with my warlock but it was very slow i just quested and helped out with the felguard since light bolt and earth shock resist ALOT on higher mobs, My mate however went from lvl 7 to 17 in NO time last night using his 70 felguard lock and boosting 2 low level warlocks. He just ran around dotting and finishing em off with the felguard fast and effective.