Editing Vehicle Design Guide (section)

=== Passengers ===
Each passenger takes up 20 bulk, the 20 bulk for the driver is already subtracted when you design or view your vehicle. You should have a gunner for every gun, as mentioned at the start while your driver can also man guns while driving, he can not reload while the vehicle is moving and his accuracy will suffer.
So you should aim for 3-4 gang members per car total.

You can add/remove passengers from anywhere you can view your vehicle, Jake's, the squad- and vehicle pages.

After adding passengers to your vehicle you can set who will drive by clicking the 'Drive' link next to the character's name.  By default, the first character you put into the vehicle will be the driver.