@Leslie757
What a cute place! I'm going to assume when you say overnight visitor, you simply mean a second guest and not that the guest is picking up random strangers and bringing them back to the property.
As Fred mentioned, I would probably start with simply basing your pricing on 2 guests instead of charging per guest. It will make things a lot easier for you and the guests!
This is something you can address in your welcome message as well. Simply ask the guest after they book if they are booking for one or two people. With my listing people will often put 2 guests coming, but will not list the name of the second guest. I will then ask them to provide the name of the second guest for my records.