The new offer can be seen in the feed of the Google Pay app. Similar to other Google apps, Pay has "cards", which give you various info and one of those happens to be regarding the referral promotion. The card is conveniently called "Invite friends, get up to $100" and has a blue "Invite friends" button. Upon pressing it, you'll see your referral code, which you'll share with referrals via the "Invite friends" button (yes, there is another one). When they install Google Pay, first-time users will enter that code and when they make their first purchase you'll both receive $10.
The referral program is available in a number of countries, including Australia, the UK, Russia, the US, and more, but keep in mind you need to be 18+ to participate and you have 28 days to spend the promotion money in the Google Play Store. If not, they will disappear, making all your efforts worthless.
There's also another promotion regarding Google Pay. It involves making purchases using the app on 5 different days and each time you do it, you'll be granted $10 for a maximum of $50.