Well.. test it.

Have one toon kill something ungrouped.
Then have that same toon group with a linked account, and kill a mob of the same level.

Assuming it was a green or higher mob.
And the toons are 1st-59th.
And within +/- 4 levels of each other...

You should receive 200% extra, for a total of 300% (3x) normal experience.
Plus a little more than that, for the party bonus.