There’s a chance that this guide might not work for some apps. The reason being some developers choose to create closed beta programs and you cannot join unless you are specifically added and invited through email.
You can join beta programs and download the app without waiting for any confirmations. I was able to install Runescape on my device even after the Beta program was full.
Simply by following these steps:
Copy the ID of an app. For example:
Head over to this page:
https://play.google.com/apps/testing/[APP ID HERE]
And join the program.
You can now head back to the Play Store app’s page from where you can install the app on your device.