The downscaling per person hitting a mob is 55% XP, so from 11k, it goes to around 4,8k. 2 others, 2k. 3 others: 1k. 4 others: 500 XP. 5+ people: 19-200 XP.
This means that, unless you are on a COMPLETELY DEAD server, you get literally fuck all XP from these now. And what's worse, people trying to help others, works against the people they are trying to help cuz it cuts thier XP gain in half.