I've driven about 35 miles w/ 4 people in the car - myself; wife; step-son & daughter-in-law. Mostly highway. The car felt fine and didn't seem substantially down on power. We had about 700 lbs of passengers & baggage in the car at the time. I think there was room enough in the back for the kids, but they aren't overly large (5-9 & 5-3). They probably could have handled an hour or so more w/o too much complaint if necessary.
ive driven with my GF and one of her friends which no joke weighs about as much as 2 people. The car is affected by weight very easily since its so lightweight to begin with. Definitley hurt the performance of my car when I have others in my car
In all seriousness, I had 3 of my friends in the car (4 mid-20s guys, 170-230 each) and it was fine. Sure, you lose the rear-view mirror when you have people sitting in the back, but I never felt like the car was out of its element.