I have an idea. When someone leaves on the roulette, why not double the bonus rewards for the next person joining in? This would make it more likely that people would suck it up and run it more often after replacing them. Or if you get a roulette that you absolutely hate you have the option to get double reward if the party votes, so you ask party to vote on you getting doubled rewards (like a bribe :P)to that dungeon so you can stay? (only once per person per day on the roulette)

Just an idea..