While "XP is simply shared", when in a group with characters that are of wildly different levels (say 5 and 10), sometimes it seems like the high level character is quite effective in suppressing experience gains from the lowbie.
In another first-hand experience last night, two fresh level 1 characters started playing and included a level 3, but they all dinged level 6 at nearly the exact same time (all while fighting a given group of mobs). What is going on here?
Occasionally I've seen people say when they join (us lowbies) that they won't get experience if they don't attack; which if true may explain it if they kill them before others can even cast their spells (despite being right next to each other), but I have no proof of this. If this was the case though, classes that only did close AoE/single target damage might be screwed when playing with bigger AoE classes.
Best Answer
If players are within 4 levels of each other, experience is worth 100%. Starting at 5+ levels apart, experience decreases by 10% each level until it reaches 10+ levels apart, it then decreases by 95%.
Example: a level 1 being boosted by a level 60 will still receive 5% XP per monster kill.
However, if you legitimately kill monsters in a more difficult act with friends your own level (or solo) you receive the full amount of XP.
No solid citations yet, sorry.