My youngest son and his friends have been to Glastonbury Festival for six years on the trot. Usually, my husband and I will rent a caravan for the week, and the youngsters will stay with us in the caravan on the first night, then one of us with drop them off at the festival site the next morning, have our week away, and pick them up on chucking out day. It was a four bedroomed caravan, and slept up to 10 so plenty of room.
Last time they went, we fancied a change from the usual caravan, so we rented a small two bedroomed cottage.
I emailed the owner asking if she would mind if they stayed with us overnight on the first night, and explained why. Her only reservation was that the cottage was very small and they'd not have much room, but she was ok with us going it. She asked if we minded if she didn't provide bed linen or towels for them. In fact, they all used their sleeping bags and it hadn't crossed our minds for them to need bed linen.
So yes, I have taken extra people, but would not do so without having the courtesy of checking with the owner first - and not for the entire duration of the rental.