Just a few thoughts...
Max 6 players:
Allow anyone to join. You don't need skill tiers like instances. Each task gets harder...therefore, early mixers would only be able to complete a few tasks. A reward would be given each time you complete a task.
You don't need to lose rostos...it's doesn't have to be huge risk/reward. If you fail, you *can* offer a rosto, but don't have to. Think about it, you're doing lots of work, but you're not keeping the items...that already seems to be a loss. The rewards will vary, depending on chance and the tier level. You can keep items from each tier (only in the instance) Therefore, immediately after your team enters, you will all scout around, harvesting and gathering basic supplies (such as HE ingredients, food, ect)
You can only enter the instance 1 per week or every few days...this gives more incentives to win and to offer up a rosto to continue playing. Really, it's like an old arcade game...you die or don't beat the level and have an option to put 50 cents in the machine...just with rostos.