A recent high potential incident occurred on an offshore rig during a normal crew change which was the result of a number of procedural and behavioral failures. A helicopter was on deck and the passengers and cargo were offloaded without incident. The HLO requested and received permission to load some of the manifested baggage and cargo inside the helicopter as the baggage area was insufficient. Once this was accomplished the HLO commenced the boarding of the inbound passengers. The manifest listed eight (8) passengers. The pilot noticed movement of the port crane boom as the Crane Operator attempted to make a lift from a supply boat alongside. The pilot notified the HLO of this and the HLO left the helideck to shut down and secure the crane. After the Crane was shut down the HLO returned to the helideck and noted that all the doors of the helicopter had been secured. Following a survey of the surrounding area the HLO gave the clearance signal for the helicopter to take off. The return flight to shore was completed uneventfully and as the pilot prepared to give the approval for the disembarkation of the passengers, he noticed that one of the passengers had some of the baggage on his lap secured by a seat belt. After making a head count the pilot discovered that he had a head count of ten (10) passengers and not the eight (8) as originally manifested. The pilot ordered a cross check of the manifest and requested that all passengers and cargo be weighed. It was determined that the helicopter had made the return flight with approximately 308kg or approximately 690 pounds of weight in excess of the aircrafts maximum allowable limits.