I like your idea..However the main issue i see is how are you going to attract the users....If you use some kind of initial reward then the set up cost will go up and there will also be the issue that users can join to get the reward and then leave the app...The app have to be really engaging for the user to stick to it. Moreover as per my personal experience, there should not be too much advertisement in the beginning or the user will get annoyed too soon... I wish you the best of luck and hope that it works...Pl keep a capacity growth model in mind as I have seen a couple of apps/startups that took off really well but then couldn't manage the increase in their user base.