You should be able to share a google sheet only with certain people that have google accounts instead of publicly. I think you can then set permissions for each individual.
I’ll also add that depending on what your app is intended for, there are ways to have content that is view only by everyone and content that can only be edited by the original poster. If you want several people to edit several posts, then you can either use the combined array column of Per User Data, or some sort of tab with a list item that only certain people have access to that would take them to an edit area. Kind of like the Admin demo in https://concepts.glideapp.io/