I got into the game a couple of days before my best friend. My best friend got in a couple of days later and around level 9, me and my other friends decided to let him run a duengeon with us. He was level 9 (or maybe even 10) and we were either 16 or 17. Whenever my friend was in a party with us, his xp got destroyed. So not only could we not help him catch up, but even being in a party with us, practically negated his xp gains. ESO is not friend-friendly and that makes me sad because I love just about everything else about the game (aside from bag management but that's a gripe for another time).
So my question is: How many levels ahead can you be from other party members, before joining a party with a lower level person nerfs their xp?