In https://developers.facebook.com/ at the top right, hover over 'My apps' and click onto the app you begun creating as demonstrated in step 1. Once you have entered your app overview, click on 'Roles' in the menu on the left.

Once inside of the Roles section of your Facebook app, you will see that you are an Administrator of the app as you created it and have all the permissions. In this page, it is also possible to more people from your team with differing levels of permissions. It is recommended that you add at least one person as a tester so that the app isn't only tested by you.
​If you wish, to add more people with different permissions, hover over the '?' next to each role's area (seen in the image below). 

1. In the section, 'Testers' click on the button 'Add Testers' (as seen in the image above), this will provide you with a pop-up window where you can begin to add testers for your app.

2. It is possible to add as many testers as you want to the app. Simply type their name into the field and hit enter.

3. When you have added all the people you wish to be testers, press the 'Submit' button in the bottom right of the pop-up window. You can remove or add more testers at any point.

Please note: you will need to be Facebook friends with all of the testers you wish to add.

The messenger functionality of your page will require your testers to provide a group code to test all of the features. They can join the group you connected, or use the group code, 'MessengerScreencast' - this group is used by the reviewers at Facebook in the next step.

Once you have tested your Facebook page's messenger functionality you can progress to the final stage, which is submitting for approval to enable this feature to the public. This is described in detail in the next step.