It is not possible to get which users are watching a stream.
Extensions wouldn’t include any user who doesn’t opt-in to your tracking, or is watching in any of the numerous ways where the extension wont be loaded.
Viewership data is private, and for privacy reasons is not accessible to any 3rd party with the exception of Game Developers and even then it’s still limtied to users who are watching a game by that developer, and both the viewer and channel have opted in.