Test Android app with selected users

I am developing an Android app. During development, I want to push the app several iterations to 50 selected user so that they can test the app for bugs etc. These update should happen automatically. I know I could send emails with the .apk file to the selected users, but this is not an option. The update should happen automatically, same like if the app was downloaded through the Google Play Store.

Basically something similiar as Testflight.de or Fabrics.io for iOS...