When the duty finder pops and it asks you to join or withdraw from the duty, if it's an in progress duty, it should both A) tell you how many people are currently in the duty and what roles they are, and B) not count as a withdrawal if you decide not to join.
This would be especially useful for DPS, who get long queue times only to join an in progress dungeon and end up with only one person there and have to go queue again and wait for another twenty minutes. The current system has the potential to waste a lot of time