At the end of the flight, the pilot selected a field to land. The pilot had the front seat passenger open his door to check for ground clearance as the field was covered with 12-15 inch high grass. The passenger reported all was clear and the helicopter settled to the ground. As the rotors were still winding down, the rear seat left side passenger exited the still open door, walked to the rear and under the tail boom. The passenger was then struck by the tail rotor. The pilot was unaware that the passenger had exited as the pilot had not given the OK to exit to the passengers. The pilot felt the helicopter shutter and heard a loud noise. The engine was then immediately shut down and the pilot and passengers exited the helicopter. The passengers had been briefed as to the areas to avoid around the helicopter and the passenger that exited the helicopter had been reminded at least three times during the day to stay away from the rear of the helicopter.