First, you would enable additional users for your app via the App Console. It sounds like already did that, based on the "0/500" figure you shared.
Then, you would distribute your app to your testers to link to your app normally, as if the app was already released for real.
Based on your description of "they have successfuly selected a file from dropbox. and still the linked users counter shows 0/500" it sounds like your users are already using your app. If that number isn't increasing, and your users just "select a file", it sounds like you're using the pre-built Chooser or Saver functionality. Using those don't qualify as linking users, and in fact you don't need to apply for production if you're only using the Chooser or Saver.
