There's currently a XP exploit available in greater rifts being used by many top lvl paragon and streamers. Posting here so everyone can use it so it can be fixed soon.
I used it myself in seasons and got paragon 1-277 in under 4 hours. Gabynator and many others are using this as well to boost their paragon lvl.
Exploit consist of the following :
Make a greater rift lvl 27-36 (the one you can clear faster) with 4 players(can be done with 3 as well). Player A will be the leecher. Players B-C-D will clear the rift.
Once the rift is opened, Player A will zone in with the group to kill 10 + mobs. after 10+ mobs he will port to town and stay there for the whole rift while players B-C-D clears the rift. For some reasons, you get 50x+ more xp while in town than while in the rift. You get even more XP with an XP pool and +XP gear (hellfire ring, leoric crown with ruby, etc). Once the rift is cleared you switch roles.. Player B becomes the leecher and players A-C-D clear the next rift.