4 people in the backseat?

As a new driver with my temporary license, I'm quite looking forward to receiving my full license in 6 months. I currently own a Ford Escape (small SUV) that has a wider-than-most rear seat and a flat rear floor so everyone can easily ride in comfort. I'm in a closeknit circle of 6 extremely close friends and we routinely go places together. My only question is whether or not it is legal to fit 4 people in the back seat if they are all buckled? We have a seat belt extender used when we had a car seat in the back seat and therefore, by utilizing the extender we could comfortably fit all 4 into the back. The question is not regarding comfort, however, it is regarding the legality of this. Is it legal?

As G said seating as per the vehicle. If you break this law you won't have your license for very long and you are not doing your friends any favours risking your and their safety.

If you drive with over capacity, you need to get a vehicle that will accommodate all passengers safely. In other words, get a van or an SUV. Alternatively, you can just ask your another friend to drive all the remaining passengers. Otherwise, it is a no go.

As a driver, you need to follow your vehicle manual. And all vehicle manuals say that a seat belt needs to be worn properly by all passengers.

Using one seat belt for two people is very dangerous. The two people sharing the seat belt would smash into each other. An improperly positioned seat belt can kill a person. The lap belt needs to be on the person's hips and the shoulder belt needs to be positioned between the neck and collar bone. An improperly positioned seat belt can cause severe and permanent injuries: http://www.boosttil8.org/

No, it is not. Count the number of seatbelts in an given vehicle. That is the total number of persons you can legally have in the vehicle. Period. No double buckling.