Here's a simple method for getting millions of experience on your alts each time you finish a rift with a level 70 character. I believe this works for turning in quests to get horadric cubes.
First, complete a level 70 rift on any torment or difficulty setting. Next, take turns logging off and getting on a lower level character. Make sure that at least one person is in the game at all times so it doesn't go away. Lastly, turn in your quest to give everyone a big chunk of experience.