Apple's person-to-person payments feature, Apple Pay Cash, is now available in beta form. The feature lets users transfer money within the Messages app on iPhone. You can only receive the capability right now if you've entered the iOS Public Beta program.
Once you've updated your software, an Apple Pay button will be present in Messages' apps section. You can use that button to start a payment, though the functionality can also be initiated by asking for money in a message.
Payments will be funded using the credit or debit card attached to your Apple Pay account. There's an "industry standard" fee for credit cards, but no fee for debit cards.
The beta for Apple Pay Cash is limited to people in the US with iOS 11.2 or later.