Boosting with multiboxing

Not sure about what you suggest, but you could make an end-boss game with your main and invite the level 1's in - they complete and now have access to story mode and I think torment levels

You could even have them leave the game after being invited in - have of them resume it, and invite everyone back in for a level 1 boss fight - I THINK :)

Just do the Act IV: "Clear the Hell Rift" bounty runs. It's the fastest EXP right now.

Asuming you have 4 accounts with at least 1 lvl70 char in each, and u want to boost new chars on each to 70:

1. Create a Torment VI game on each lvl70 char (no party).
2. Check on each character if u have the quest (just mouseover the list of bounties for Act IV).
3. If no character has the quest, leave game and create a new one until you get it on any char.
4. Once u have the quest, have all other chars join that game.
5. Leave game on all the characters (max 3) u wanna power level and switch them to low level chars, but leave them in the lobby.
6. With the lvl70 left ingame, rush thru Hell Rift lvl1 (dont kill anything) and go lvl2. Clear everything but a couple white mobs. It's very easy since all are white mobs. Every once in a while u will get a purple unique boss which is not hard to kill, but if u are too weak, u might wanna lower the difficulty.
7. Once u have just a few mobs left, join game with the 3 chars u wanna power level and teleport to the lvl70 char.
8. Kill the last mobs in lvl2. That will give u the bounty reward for all your chars. With pools at lvl70 its 80m exp and like 420k gold.

I've done all my chars/classes like this in just a day. It's very easy and fast to do. U gain a ton of exp and also gold. Sometimes u can get a Cursed Chest which can be as easy as killing 5 white mobs or 3 elites. If u get the elites, just leave game. It gives like 20m more exp.