User not showing in Unique app user emails

Current situations:

Have deployed an app, have added several domains, require user sign in/authentication, and am allowing Any Provider for authentication. Recently a user from an authorized domain has installed my app and has submitted data via a form in the app. However, his user email is not showing up in my list of “Unique app user emails”. Where can I find a list of everyone who has installed and is using my app?

So if you know a time stamp you can use the audit history of the app to find it. What is this Unique app user emails?

Please contact for help with this.