Hello, Libi!
I have a party on Collabra that usually kill the raging mage. In order to do so, we divide so each of us kill 250 yielothax and gather together to kill the boss when it gets mortal (after killing 2k yielothax).
But after seeing your question, I decided to test if, in a party that shares experience, would be possible to count the 250 kill counter for both of the players on the party. You can check on this first two screenshots the results:
As you can see below, we killed together 300 yielothax:

We decided to kill them by making the second player (Daslei Errepe Otraveiz) trhowing some avalanches in order to make sure that I would be the one to frag every one of the yielothax (you can see above the difference in damage between us). As you can see below, he used very few supplies during the hunt

After we finished the 2k yielothax on server, we made the boss mortal and then tested if he could get up on the floor

AND HE COULD!
And to be sure that he didn't have any yielothax killed before so he got it accumulated, I asked him to print his bestiary after the 300 kill (in fact he participated on 299 since I one-shoted one yielothax)

In resume: Yes, it is possible to share the kill counter as long as you share experience on the party. Hope I helped you out, it was fun to make the testing and be proofed otherwise (I really didn't believe it would count).
A special thanks to Hashimuerta, who used his maker to help me test this out