For some time now Google has allowed developers to release alpha and beta builds of their applications. All they would have to do is build a Google Plus Community or a Google Group which users would join, sign up for the beta/alpha build program, and then they would get these updates on their devices via the Play Store just like other updates. Facebook has used both beta and alpha build features when allowing their users to get early features and now they’re bringing this functionality to their messenger application.
You can sign up for the beta program here. Once you join the Google Group you have to go join the beta build program on the Google Play side. Finally, all you have to do is visit the Play Store via any of your devices that are connected to your Google account and you can update to the beta build. While you will get new features before others, you can also come across a lot of bugs that might actually break the application. You have been warned.
Join Messenger Beta | Messenger Google Group
Source | Facebook