Obviously you cannot boost 5 at the same time, when defining "boosting" as a way that a high level character is pushing you through instances.
Blizzard nerfed pretty much all possibilities to abuse the mob tagging system in the open world.

You still could just play your 5 man team and using natural or quest related hyperspawns or simply do instances; you still have a severe advantage when it comes down to just grinding your way up.

Second option would be to boost a 4man group twice, while the 1st team includes a mage that's pushing the 2nd, leaving you with 2 60s on 3 accs and 1 on 2 accs.