Lets say Driver A is driving on the track and you plan to swap with Driver B
Driver A goes into the pits, stops at a box, and only until that pitstop is finished can you swap the drivers.
To swap, both drivers need to do a sort of an 'agreement' with each other. Press 'N' to bring up the connections list and beside the "SS" buttons you normally use to send setups, you should see a "T" I think it is. The person planning to switch should press it first, then Driver A that is in the pits should press his "T" (which is lit up) to 'accept' the switch. You (Driver B) should then appear in the car, and Driver A will be spectating . And off goes Driver B!!