Just completed this in a group. Not sure how to spark the quest while in a group, it seemed random, but we all got the quest done and received our titles.
Some helpful tips- You can die and lose your bounty, just don't release and run back to your body. If you have some one revive you (through DK or Druid BRes, even warlock seemed to work, standard revives work too), you don't fail the quest and can keep killing people for credit.
It did look like we needed to tag each person that died in order to receive credit.
It took me ~40 kills to get the quest, dropping group after becoming an assassin and killing some one triggered my quest. We were all able to be on the quest at the same time.