Some of those are four berth - have a seat across the rear that folds down to make a bed. Its probably they had sitting 3 people in the front and 4 in the back. I can easily imagine 6 seat belt (3+3), but unlikely to have 4 across the rear seat.
I am finding it hard to judge them, I remember my youth with a bench seat Mk2 Cortina regularly carried that many of into town on Friday nights..... and would not have hesitated to do the same as these (presumably) young people were doing.
Don't be alarmed when you see me talking to myself, I am getting expert advice.