I was playing on a level 26 HC character the other day and a friend of mine asked me to power level him, with neither of us really sure how XP mechanics work in this game. He was level 12 so it was a 14 level gap. I didn't think he'd get XP, but we grouped up in Act 1 and he got a few levels at what seems like a normal speed.
So how do level gaps affect XP gain for both the higher and lower party member? And what about the level of the zone compared to your own level? If you bring someone to higher level content, will they level faster there or is there some mechanic in place where they get diminished XP? Lastly, how much higher can you be in a zone and still get XP? Like, if a zone is level 20, at what point do I need to move on?