I'm not sure what you mean by "runs an app for all users", Are you writing a separate app that you want the hangout to automatically open on entry? Doesn't it make more sense to do this the other way around?
The app runs within Google Hangouts (like drive, chat, youtube, effects) which is part of the draw of using that platform.
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.