During a cross-country flight, the pilot and a passenger in the rear seat unbuckled their seat belts to try to retrieve items behind the pilot's seat. An adult passenger seated in the front right seat, who was unfamiliar with the side-stick controller in the airplane, grabbed the side-stick controller while adjusting their seat. The pressure on the control stick caused the airplane to violently climb and descend 3-4 times before the pilot was able to regain control. During the excursions the pilot and the back-seat passenger bounced off the ceiling, resulting in minor injuries to the pilot and serious injuries to the unrestrained passenger. The pilot declared an emergency and proceeded direct to Chicago Executive Airport where he landed uneventfully. The pilot stated there was nothing wrong with the airplane prior to the event occurring and the airplane was not damaged.