How much does an extra passenger add to a car's fuel consumption?

Not measurable. I have records of my travels with and without my wife. The miles per gallon performance is relatively constant. Speed and wind affect miles per gallon more than one passenger. In fact, I just completed a 5,300 miles road trip. With bags, cooler and supplies. Plus the spouse. The same MPG as with no cargo. Only the wind and it WAS WINDY in May in New Mexico and Arizona, over 35 MPH at times made a difference. At times, 5 MPG less. In still air at 65 MPH the car uses a lot less fuel that 65 plus 35= 100 MPH. Yes, for many miles the wind was directly against our travel direction. Of course, when the wind was pushing us, the MPG was 5 MPG MORE. It is a hybrid SUV. Nice question. Accept the ride with grace and if you think about this a few candy offerings will convey the feeling. Good luck.

Depends entirely on the drive, the vehicle, and especially the tires.

Are you driving up or down hill? Added weight means a lot. Flat land, added weight within reasonable limits is almost meaningless.

Some vehicles don't do well at all with a few people in them as the powertrain is calibrated to move a lightweight car over nearly flat land, will be far outside of its peak performance with an extra 2-3 passengers plus gear.

Tires are one of the biggest factors (road conditions too) as cheaper tires tend to distort more with increases in load. Most passenger car tires are on the cheap side and make a lot more contact with the road this creating more drag, more wear, and more fuel consumption.

Besides the technicals, why would a driver want to give you a ride? Are you good company? Are you muddy? Is this a planned ride or just hitching? Usually if it is a distance you might consider walking as an alternative, the fuel usage in negligable, but you should definitely show thanks (cash).

It's obvious that you and your bushwalking buddies are the worst tippers at the restaurant, what about the wear and tear on the vehicle? The insurance to drive it so your tree hugging buddies can sue for injury damages if there's an accident on the way to the bushwalking convention and you get hit by an uninsured motorist? I could go on but why, it's futile with that self centered mentality.

The formula = The deadbeat walks, it's in the funk & wagnall.

No, the extra weight does not change wind resistance, which is a major factor in fuel use at highway speeds.

I think it's every 100lbs. extra weight offsets fuel consumption by 1mpg.

not much, just an extra sack of potatoes. there is probably an online MPG calculator you can try.