People keep building that app, using it to acquire customers, then pivoting into a generic payment platform because payment processing is a volume business and consumers splitting checks is such a tiny percentage of the market. It's the founding story for PayPal (originally for PalmPilot!), WePay, etc.

If you really want to make sure everyone contributes, try Tilt maybe? I think the idea is that the pool can't be used until everyone pays in. https://www.tilt.com/

For mobile payments, Venmo perhaps? https://venmo.com/ Though I would probably just use PayPal myself.