The way the XP system works is kinda funny.
So the person/group who gets the tag gets the xp based on the dmg they have done (it's exponentional, but I will use a liner example).
Lets say you burn a mob 60% of the way and a lvl 60 comes and 1 shots it, you will get the majority of the XP we will say 60% (it's more, but this is an example). It's proportional to the person who did the most dmg vs all other dmg done to it independant of each other.
Lets say you burn a mob 60% of the way and 2 lvl 60's (not grouped) each hit it once for 20%. You will get 80% of the XP.
Lets say your lvl 40 and you tag the quartermaster outside BWL. 40 people waiting to raid (not in the raid yet) hit him once. You get an INSANE amout of XP. 5500 was the most I got for 1 kill and I recived several 3-4k's.
But these 2 should be proof enough.
Gerr photo bucket compressed the images
LOL, I just noticed you can see my XP bar progression like a flip book
Get friends to come help you level, you get they tag, they stay *ungrouped* and as long as no 1 person is doing the majority of the DMG, you get fast XP. I recomend mobs ALOT higher than you and the more friends you bring the more XP you will get.
Note: More people, higher level mobs, each person doing less dmg are the keys.