To make sure every updates and features are acceptable by their users, WhatsApp needs to have users to confirm it before make it available to all users by updating the official release or the stable version release. That's the job for WhatsApp beta tester users.
WhatsApp beta Tester users will get the features first before it release to the stable version. They also can report the feature if it have some bugs. The great thing is, you can use it before the others, before your friends even know it there.
To become a WhatsApp Beta Tester, you just need to simply open up Google Play Store and scroll down until you find a section says "Become a beta tester". Tap on "I'm in" button and confirm to join beta tester program by tap on "Join".
After doing that, all you need to do is waiting until the beta sign up progress completed. Then you need to update your WhatsApp be able to use WhatsApp Beta. Take a look at this short video below or view it on Youtube by this link https://youtu.be/4XOnMmsOV7I.