I've done something similar to this before.. Let's say that your guy object is of the class Guy. And your car object of the class Car.
Your Car can have a variable of type Guy named driver.
public class Car extends Actor
{
private Guy driver = null;
// More stuff here....
}
And you can write methods in Car to load and unload the driver. And in act(), if the driver != null, you can follow keyboard commands to drive around.
Having the driver accessible by the Car means that you Car can also pass along damage, powerups, etc to the driver and other fun things. :)