A joyrider is the person who goes on a joyride in that crime sense: they take a car that is not theirs and drive it fast and dangerously for fun. The trip is the joyride; the person is the joyrider. You meet the word in news English and in talk about crime. It is informal-to-neutral, not a legal job title. Though actually, some speakers stretch it the same way they stretch joyride, and use joyrider for anyone driving in a wild, showing-off way — even in a car they own. That stretch exists, but the filed meaning is the person who steals (or takes) the car for the thrill. One joyride can have several joyriders if they are all in on it.