I have a product owner for one of my sprits who likes to involve people outside the sprint in sprint tasks. I've attempted several times to explain how this isn't ideal, and in fact slows down the sprints and causes confusion.
I recently saw that they invited their entire team (about 5 people) to the next sprint planning meeting. They didn't ask me about it (me being their scrum master), and I assume the reasoning behind it will be something along the lines so they can "see the process" or something to that effect.
I want to explain to the product owner that having people from outside the sprint in sprint planning is a bad idea and can cause confusion. What is the best way to explain to the product owner that adding people to this meeting isn't a product idea?