When you watch someone get on the quad bike. Once they are on the character models are laying down on the quad bike. If someone gets on the back they too are laying down.
Once they drive off watch the handle bars. You can see that the player is controlling it.
{F17969} {F17970}
Face a quad bike in multiplayer
have 1 person get on the bike as the driver.
You'll see the character model laying down with the head at the handle bars
have a second person get on the quad bike
There character will be in the exact same position.
I can confirm that - happened to me today on Wasteland server. Looks rather inappropriate :P