Nice the division short trick:power level friends via high level side missions
If you are a high leveled player and have friends that are lower you can have them party up and do side missions with them and have them receive the full exp for doing it.
What I did was go to the safe house in a higher level zone, have then fast travel to me so they unlock the zone. Once your friend is at the safe house then they can go to the board and grab missions. Once they do then set off to do encounters and missions with them in the group. The only hard thing is making sure that they stay back and avoid dying.
This works best with two higher level players to help carry the lower level player. Via this method we got a friend from 4 to 12 in just a couple missions and encounters.
This is also good for when you are doing side missions for yourself just to level and gain experience because they also get to benefit from your hard labor :D