The feature appeared in the Southwest Airlines app on Monday, a day earlier than the announcement, but allowed me to add my boarding pass as I traveled to Google I/O to my Google Pay account.
The process is similar to that of Apple's Wallet app. When viewing my boarding pass in the Southwest app, a button was present to indicate I could add my boarding pass to Google Pay. Tapping on the button asked me to confirm the addition.
Android P: All the new features and tools announced (so far)