Adding reward for completing an event with someone who hasn't beaten it yet was a step in the right direction. You could also argue that people would use that reward check to see if anyone hasn't beaten it yet. The good thing is that the system isn't telling you who is the person that hasn't beaten it yet so the leader could probably just roll with it instead of spending a long time trying to find out who beat it. Achievements and linking items however seems to function to lock players out that hasn't beaten it yet.

Instead of 100 myth, add another slot of coil items if you have a new player.