There are two things :-
1. Shared expense tracking is still a broken problem which some of the guys tried to solve.
2. I really like the way whatsapp simplified user onboarding.
I combined the two ideas to create the app. This problem was something close to be for a long time. I am fairly deep into ideating & mocking up (https://dl.dropboxusercontent.com/u/76443013/Expensio.png ). The weservices part is ready too - http://expenseclub.herokuapp.com/transaction/1/users
I am looking to get the 'usable app' done in a day. I run another startup (www.codelearn.org) so probably would not be able to pursue this full-time but I would really like to see this problem solved as I am the one who would be using this app regularly with friends.