Open the payment in your transaction history and tap Split bill, or go to Get Paid → Split bill for a bill you didn't pay with Ziina.
Choose who you're splitting with: your Ziina contacts, your phone contacts, or a number you add yourself. Then choose how to split: equally, by amount, or by shares, and tap Split payment.
Friends with Ziina get the request in the app. Friends without Ziina get a text message with a link to pay.