We just added something that people have been asking for for a long time. You can now restrict access to your application with a username and a password (which are independent from the concept of a user for your app). If you go to settings, general, you can check the box “Limit access to this app with a username and password” and pick a user name and a password that you’ll have to share with people you want to be able to access the app.
This feature is available on the Personal plan and above. Keep in mind that if you activate this feature, getting feedback and help will be tougher.