The Testers page is where you define a list of users who you want to be able to test your game, even when it is hidden from the public view.
This option is only relevant for games that are currently being tested. Once public - your game can be viewed on the CurseForge website by anyone.

To add a new user to the list, simply click the “Add tester” button and you will be prompted to enter an email address.
You can have up to 10 testers at a time, but you can always talk to us if you need more.

It is important to note that you can add ANY email address, but the user testing your game must be logged in to the CurseForge website with the same address you have defined in the list in order to see your game.