If you need to lookup users by uid, email or phoneNumber, you can use the Admin SDK to do so:

https://firebase.google.com/docs/auth/admin/manage-users

You also even have the ability to download all your users: https://firebase.google.com/docs/auth/admin/manage-users#list_all_users

You would need to do that from a Node.js backend server or via HTTP endpoints with Firebase Functions.

In addition the Admin SDK allows you to set custom user attributes which could be helpful if you want to create different user groups:

https://firebase.google.com/docs/auth/admin/custom-claims

admin.auth().setCustomUserClaims(uid, {groupId: '1234'})