Here's how i got to lvl 70 in 6 hours (i wasted two to three hours on bounties and rifts) (There more people you do this with the faster it goes up to 4 people ofc)
Since you have adventure mode unlocked from the start lets use that to level up fast.
Make a game with the Torment 6 difficulty... We wanna look for some sweet exp from bounties
You wanna look for these three bounties
Act 1: Cemetery of the forsaken: The jar of souls
Act 2: Howling Plateau : A miner's Gold
Act 5: Westmarch Commons: The Miser’sWill
All these bounties can be completed without ever having to slay a single monster.. WOOOH!!
I recommencd doing the act 2 and 5 bounties only.. as they give twice the exp as the jar of souls and can be completed Quicker
Since i did this i learned a few good things from this, so here's some tips:
When doing the jar of souls, just run down and find it and activate it, no need to stay alive as soon as the timer runs out you just have to click the jar to get the exp
When doing act 2 have everyone else wait at the waypoint in the area (so they get the bounty exp) and let one person go complete the bounty, the miner wil only progress when someone is alive near him, so just kite around, if you die, doesn't matter. The progress will stop and wait for you to get alive, though if someone else is near and you die the bounty might be completed while you're dead
Act 5 i found it best to just find the house with the chests, run in and die to the elite pack, let everyone on the team die and ress on the same time, and open the chests before the elites can kill you again