How to become a beta tester for Spotify on Android

First things first, you have to head into the Spotify beta form page on Google Play.
When you get to the Android App Testing for Spotify page, you will see a button with the same 'BECOME A TESTER' label. If you have already been signed in, it will show up right away.

How to leave the beta program

Leaving the beta program is just like signing up. You have to follow the same instructions written above, but instead, hit 'Leave the program' option.

Spotify is one of the most popular music streaming platforms in the world right now. It has more than one million installs on the Google Play Store. The beta testing program of the app is restricted to some countries, but it's currently undocumented where it is available. We have some info that it's not open for US users (please, confirm) but it is accessible in Germany. We are asking you guys to try and confirm if it is available, or it isn't in your countries.

Btw, here is a list with more info about the beta and Spotify in general which you can access from this link.