Public beta program for Google Messages kicked off with a build that made the app crash
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Android Police reports that earlier this week, Google allowed Android users to sign up to be a beta tester for its Messages app. Many of Google's core Android apps offer this. To sign up to be a beta tester for apps like Gboard, Phone, Google Search, Google Maps, and now Messages, you need to have these installed on your Android phone. Once that has been accomplished, open the Google Play Store, swipe to the right to open the menu (or tap the hamburger icon at the left of the search bar) and tap on My apps & games. Click on the Installed tab and click on the name of each Google app to see the listing page for each one. Scroll down and right under the Developer contact information, you will see an invitation to become a beta tester.

Android users can now sign up to be a beta tester for the Messages app
Other Android apps like Instagram, Netflix and WhatsApp also allow you to become a beta tester using the same method as described above.
Things that are NOT allowed: