So I was leveling my 40 warrior to 41 in Kraag 4 solo and got bored so invited a friend. I always watch xp per run, and was very surprised to see that I got no bonus from having a second person there.

When I wrote the Kraag 4 guide last spring, I did extensive tests and calculations regarding xp gains with elixirs, elixir stacks, and parties. This is a quote out of the xp guide I made at that time for my guild:

Party
2 People: + 33%
3-4 People: + 45%

You do not have to be actually "in a party" - xp gains appear to be the same just having others in the same zone. XP gains appeared to be slightly more with 3 people than 4, contrary to STS' comments in the past.

Important: You must remain in close proximity to share xp.
If you can't see them on your screen, they're getting too far away, and you will all lose.

So you can see the increase (when it was working) was quite substantial, so it's not just a little bit off - the bonus is missing completely. I did not get a chance to test with a party of 3 or 4 this time, but if the bonus is missing for a party of 2, then it's likely missing for all.