Not sure if this is widely known yet, but a few guild mates didn't know about it so I figured I would post it here.
Normally, when you try to do those high level xp missions (10k-20k) with a lower level follower, you get diminished returns. For example a 10k xp mission: A level 100 follower will get the full 10k for a successful mission, but a fresh level 90 will only get around 1k due to his lower combat effectiveness.
Harrison Jones has a perk called Mentor, which gives every follower on the same mission as him his combat effectiveness(copies over his level and ilevel). Not only does this help you get higher success chances on missions, but it allows low level followers to get the full xp reward from your mission. Because your low level followers are now raised to Harrison's level, they no longer get penalized. I've leveled tons of followers from 90-100 in 3-4 missions using harrison jones. Its really handy for those inn users that are recruiting a fresh 90 each week.
You get harrison jones by obtaining this achievement.(Do 6 of his daily quests in your garrison)
I also made a video about it if I didnt explain it clearly enough
good luck!!