My babysitter brought a friend over to my home while she was watching my children without asking. How do I ensure that she asks me first and this doesn't happen again?

Have a conversation with your babysitter the next time you see her. Start off by saying that I know you had a friend over yesterday while watching my children. It is a rule of mine that you ask me first before having a friend over while watching my children. I don't know your friend. Can I meet your friend? Your babysitter will probably say yes. Your response is: great, until I meet you friend, no friends are allowed over. More than likely the meeting with the friend will not happen.