Confusing permissions

[reagle]

Is this documentation current? I don't understand it, and on an actual page I have the options to set read or write access to owners, signed-in, and everyone. Why is "owners" plural? How do I add someone to the owners set? If signed-in, then anyone with an account on hackmd.io could vandalize the page? Also, is there a way to set permissions across a book or tag?