When you get into a vehicle, NPCs will take any seat they can find, or you can assign them in the vehicle screen. You can have multiple NPCs, based on how many seats you have. NPCs cannot really ride a vehicle without seats, as they cause it to skid like crazy when not seated, and often collide with the vehicle, causing collision damage on inner tiles and taking damage from the vehicle.
Pet carrier is an item that you activate to remove a creature from the world and convert into item form. You then use the pet carrier that holds the creature again and it places the creature back into the world. By default they only work for creatures around the size of a dog or smaller, but can be modded to fit any creature.
With NPCs, make sure you don’t tell them to guard while they are standing on your vehicle and then drive off without telling them to follow again - this results in them diving off your vehicle as you move, often making you skid and/or run them over.
