There is a definite danger in passengers using cell phones when riding in a car. This is because if a person is arguing or even talking in a consistent low voice on the phone, the driver can be distracted from paying attention to the conditions around him or her. Cell phones should only be allowed in cases of emergencies.
I do not believe there is a danger in passengers using cell phones when riding in a car. This would or should not impact the driver any more than the passengers themselves. Another person in the car with a cell phone should not distract the driver to the point that they are incapable of maneuvering the vehicle.
Unless they are actively trying to involve the driver in what they are doing, or playing their music distractingly loud, I can't imagine what danger would be involved in a passenger of a vehicle using their phone. It couldn't be more distracting then having a conversation or driving noisy children around.
I do not think there is a danger in passengers using cell phones when riding in a car. I think the only danger would be if they were being distracting with it, like showing it to the driver, or making a lot of noise. Aside from that I do not see any danger.