Finding a parking spot will be your biggest issue. Unless you want to go elsewhere to kill time or make arrangements, I would not collect the car until they arrive. You can shop around Cruz Bay until then or sign the car out but leave it at the rental lot and enjoy a bite to eat awaiting your friends.

The dock isn't big and the ferry isn't too big. If you're in the general area looking for one another, you'll find each other with no problem. Do you have a way to know for certain which ferry they'll be on? Could you maybe confirm by cellphone? Otherwise, you'd have to hang out in Cruz Bay and just keep meeting each ferry, which wouldn't be a bad way to spend some time.