When someone checks Join in Progress, it should mean they are willing to join a group in progress, not they are looking to join a group in progress. Once you establish that, don't reveal whether a group is starting fresh or in progress until you actually get into the dungeon. If you wanna quit then, great, 30 minute timer. Currently the system encourages people to look for groups in progress. You can't really expect people to not wanna do half a dungeon when their goal is just the myth or drop at the end when the game makes it easy to have that mentality.