Paga lo que comes! (pay what you eat) is one of those handy apps that you may want to have permanently installed. It's a simple and straightforward app, fitted in an brilliant and step by step interface, that allows to sum all the expenses (usually regarding a dinner, but it can be anything) and divide them among those who took part.
You must write down all the assistants to start with, and the app will help you with its neat autocomplete feature that reads from your contact list.
There are steps to include how much did everyone spent prior to the event and to tick or untick expenses according to who ate or did what.
Once all the Math is done, you can save or share your results. Oh, sure this isn't the first app to calculate group expenses, but have for sure there's been a huge effort to synthesize it all and make not the mistakes other did.
Recommended as one of those apps everybody should have installed, even though it may not seem very appealing at first glance.