Read this post first: https://help.zoho.com/portal/en/community/topic/public-forms-but-requiring-authentication
Gabriel asks for a public form accessible only after a user has signed-in.
Ruben's solution is to use the new application settings...
will let you choose the Sign in option for the users. You can Choose Google
And Yoge adds that users without a ZOHO account will get one in the background... application will be accessible after entering their gmail username/password. A zoho account for each user will be automatically created and they need not signup for zoho.
However, the new sign-in only works for private applications (right?). I have not found a way to make an app public AND require a sign-in (so as to capture the user's name, email) without some significant hacks that are probably not allowed. How does this solve Gabriel's requirement? We still need to know the email address of potential users for the private app, manually put them in the sharing module, and manually grant access to application pages/forms. A big runaround.
Once again, I see value in a third user-class: Registered Users (proposed by Gaev a long time ago). In this way, we can have an application that is accessible by authenticating with ZOHO, yahoo, Google, etc without having representation in the Sharing Module. It's somewhere between public and private, so to speak. ZOHO Wiki has this: One can set-up a page so only users with a ZOHO account can see it. Why not for Creator?