The users inside your "Authentication" in Firebase, are the user that can asses this domain. This in most of the cases can be your app user. And you don't want any of your user to be able to login inside the admin panel, right. So if you go in "scr/config/app.js" in the adminConig element you will find field allowedUsers. If it is null, every user is able to login. Make an array of emails to list who can access the data ex. ["firstname.lastname@example.org","email@example.com"]
Also there is available option for user registration. If you go in "scr/config/app.js" in the adminConig element you will find field allowRegistration and by default this option is false. By changing this option to true register option should be shown together with login option.
You can also manage who and how can manage the data using the Firebase Database Rules system.