If you try to join the beta program now, you'll see this note from Google:
"Thanks for your interest in becoming a tester for the Messages app. However, at this time, Messages app's testing program has reached the maximum number of testers that can participate in it and isn't accepting any more testers."
Don't fret, though. It's not the first (or the last) time that Google has opened a beta program.
"The Open Beta gives users the opportunity to test, and provide input and feedback, on experimental features not yet released to the general public," the company says on its support page.
It's unclear whether more testing spots will open up or whether the beta features will make it into the app itself. Google wasn't immediately available for comment.