Shortly after takeoff the airplane was observed to initiate a series of slow rolls at about 400-600 ft. During the third roll to the right, the airplane descended & impacted the ground. Exam of the airplane revealed no evidence of preimpact failure or malfunction of the flight controls. None of the witnesses reported hearing any irregularity with the eng. The plt/owner was in the rear seat & the private pilot-rated pilot was in the front seat. Dual flt & eng controls were installed. No determination could be made as to who was flying the airplane. Found inside the wreckage was a piece of paper with a sequence of aerobatic maneuvers drawn, which belonged to the owner. No comments about the departure by either occupant were made. The acft was a minimum of 31.9 pounds over max gross weight. No forward or aft CG limits available due to the overweight condition. Both occupants were wearing parachutes. A bag was found lodged under the rear seat's right rudder pedal. A storage compartment was behind the rear seat.