Back at WWDC in June when Apple announced iOS 11, it promised a new feature called Apple Pay Cash, which would allow users to send money to friends via iMessages. It wasn't ready for the iOS 11 launch in September - just like Messages in iCloud - and it got pushed back.
The feature finally arrived today, for those that are currently running the iOS 11.2 and watchOS 4.2 betas. It should automatically show up in your Messages app, as long as you have Apple Pay set up and you're messaging someone that's also running the latest beta. You also need to have two-factor authentication set up on your Apple account.
Unfortunately, Apple Pay Cash is still limited to the United States, although it's entirely possible that that's only while it's in beta. Presumably, the functionality will eventually roll out to all of the regions where Apple Pay is already supported.
You'll need to be running the second beta of whichever OS you're using (iOS 11.2 or watchOS 4.2), and that includes the iOS public beta.