The pilot asked the weights of passengers, fueled the helicopter in reference to the passengers, and then flew to a location to pick up passengers. The passengers were heavier than expected. The pilot reported that he brought the helicopter to a hover and the helicopter seemed to have sufficient power for takeoff. As the helicopter flew forward over a thicket, the main rotor rpm began to decrease. The pilot lowered the nose to increase rpm. The rpm did not increase and the helicopter impacted terrain hard where it sustained substantial damage when its tailboom separated. The pilot estimated that the helicopter weight at the time of the accident was inside the envelope for in-ground effect operations and was outside the envelope for out-of-ground effect operations. The pilot reported that there were no mechanical malfunctions with the helicopter during the flight. His safety recommendation was that he should have requested one person to stay until more fuel was burned off.