How does party XP work?

Hi, this might be a frequently asked question but i want to know for sure.

Me and my friend are grinding xp in a party(im lvl 102, hes 73)

I get normal xp from the mobs in the defiled cave, but he only gets like 50-100 sometimes. (he only gets normal amount of xp sometimes)

How does the party xp work?

Sometimes even when hes killing mobs himself, he only gets like 73 xp instead of the usual 3000 from the defiled cave mobs when im in the party. (and other places)

 

you need to reach certain damage threshold in order to get xp (i think 20% or something?)

also the xp scales by level, decaying 3% for every level difference