Most of the answers on there are stupid.
"No." Err if you're so sure please cite.
"Everyone must wear a seatbelt." not true. Only applies where they are fitted. The driver is only responsible for anyone under... some random number age, so with a car full of "adults" not wearing seatbelts it is still legal to drive.
"Ohmygod they will die!" Depends on the size of the boot and design of car.
My Discovery can have two extra seats sideways in the boot which come with seatbelts. I think I'd prefer to sit in the boot than on them.
The only thing which might be relevant is one mention that insurance might be invalid if carrying more people than the car is "certified" to carry, but there's no citation and I don't remember my car having a certificate like that. And would I have to get another certificate if I added the extra seats?

Or what if it is certified to carry 5 people and I remove the back seats and carry 5 people? Hmmmmmm. Possibly nonsense.
If someone could post a proper answer that would be interesting.