Google Workspace Add On
I would like to clarify your question with some insights.
Deploying an add-on to a local server is a feature that is not available over App Script.
In order for a user to manage and test an add-on, it would need to have "Editor" Access, this would basically give him access to the code, so it would not be possible to avoid the user to access that data.
As suggested by the official documentation, on how to share and test with developers, you added 3 users for testing under the OAuth consent screen is used for the scopes that would be used over the add-on and the project information.
An alternative would be if you have a Google Workspace account and the test user are part of the domain, to start the process to publish the application as private over the consent screen and that way is downloadable or active for certain members of your domain by following the steps on how to publish:
You can manage what application users access the marketplace over the Admin console or by publishing as "Unlisted" and sharing the direct URL of the app's store page.
I also recently discussed a similar scenario over this thread if you would like to review more about the Add-On.
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