Hunts are a great idea for people to engage in group events. However, there is no type of karma in this game to prevent griefing. My suggestion to encourage fair play is as follows:
The Named Mark should have a large aura around it so when anyone walks within the Marks aura, they will receive a stacking buff. When a certain period of time passes while the player is in the Aura, say every 30 seconds, the stacking buff increases. At the same time, the Named Mark will receive a counter stacking buff based on how long people are within its large aura.
As the stacks increase on the Named Mark, it will have a stronger defense, making all attacks weaker towards it, up to a reasonable threshold. The Players stacking buff will give them better rewards after the Named Mark is defeated. This would in turn promote a reward for people to wait before pulling, allowing ample time for others to join.