An ex friend of mine rented our holiday home for 2 weeks, I thought it would be for her, her two daughters their husbands and 4 children. When they arrived I discovered that she'd also invited another family, 2 adults and 2 children and a single friend. Also a stepson was included in one of her daughter's family. So the amount of bodies rose from 4 adults to 8 and children from 4 to 7. There were also people visiting for the day/evening and they camped out in the sitting room overnight.
I was staying in other accommodation in our garden. That's how I knew how many people were there. Also all the posts on Facebook gave the game away!
Lesson learned by me, never do a friend a favour ever again. They arrived late a night so I felt uncomfortable refusing them entry, especially with all the children, I'm sure my ex friend had factored that into her rudeness.
I think that it's incredibly cheeky and very rude to add people to rented accommodation without informing the owner, as pp have said the insurance will be invalidated.