How to notify the owner of a document?

I want to keep the owner of a document updated to changes in the doc, like when a new user joins the doc etc. Other than user() which is the current logged in user, I could not find a way to identify and notify only the owner of these changes.

The owner is also in a list of users in my doc’s users table…could I put an owner checkbox there? How would I use that flag in the Notify action?

Thanks for your help