Very First thing to do is to map 2 Keyboard keys or Buttons to enable driver swaps, To do this when in the game at Monitor screen go to settings where you assign functions to your Wheel / Controller, in here you are looking for one which says Pit Request allocate a Button or Key to this, Next is Passenger Select again allocate a Button / Key to it, in some cases it may already be allocated to P on your keyboard but check as P is also the Game Pause key.

OK How it works: 2 x Drivers we'll call them Driver 1 & Driver 2

The basic flow is Driver 1 joins the game in the normal fashion, goes right thru and commences driving the car.
Driver 2 Opens rFactor gets to the point where the car is revolving on the turntable, go to the CONNECT button then the SETTINGS button and from the displayed Menu change SPECTATE to ON now enter the game to the MONITOR screen (When entering as Spectator your name will not appear in the list of Drivers) in the Drivers list find your team mate and click on his name, Now select the P button (or whichever you've allocated to Passenger) to become a passenger, now select RACE in the bottom right corner of screen, you'll now see a message saying you are riding with Driver 1 name will appear. That's it for your part

THE DRIVER CHANGE
PIT STOP

Driver 1 Uses the Keyboard ENTER Button to scroll thru to the Pit Menu, with fuel and Tyre strategy set use the keyboard Down Arrow to scroll down to your name, now use the across arrow (Left or Right) to select Driver 2 Name and press the Pit Request button on the lap prior to Pit stopping, Now enter pit lane you'll see an illuminated red box with 2 crew members pull in here, a message will come up for the driver saying "you will lose control in.." counting down to the actual driver swap, and the Passenger will see a similar message saying they will gain control in ... At the end of the countdown the driver will become a passenger, and the passenger takes over as driver. At this point it is important that Driver 1 who is now the passenger hits the Escape button on the keyboard and views via the Monitor screen then he can go and have a Coffee or a Fag or both until his next stint is due, to change back again just click on the Race Button bottom right corner. You'll re-enter as a passenger.