PLT OF B407 TAKES OFF FROM OIL RIG OVER MAX GROSS WT.

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I WAS FLYING A CREW CHANGE FOR A DRILLING RIG IN THE GULF OF MEXICO. THE MISSION WAS TO TALK 5 PERSONNEL OUT AND BRING 5 IN. AFTER LNDG ON THE RIG; RIG PERSONNEL CAME UP AND ASSISTED DEPLANE PAX; THEN THEY HOT REFUELED THE ACFT. DURING THE REFUEL; THE RIG CLERK BROUGHT UP THE MANIFEST FOR THE INBOUND FLT; SHOWING 5 INBOUNDS. AFTER THE REFUEL; THE RIG PERSONNEL ESCORTED THE INBOUNDS TO ACFT. I DID NOT COUNT THE INBOUNDS AS THEY CAME ONTO THE HELIPORT. THE RIG PERSONNEL LOADED THE PAX; SECURED THE DOORS AND PASSED IN FRONT AND GAVE ME A 'THUMBS UP.' BEFORE TKOF; I DID NOT PHYSICALLY TURN AROUND AND GET A HEAD COUNT IN THE PAX AREA. AFTER TKOF; AND TURNING ON COURSE; I TURNED MY HEAD FAR ENOUGH TO REALIZE ALL THE SEATS WERE FULL. THERE SHOULD HAVE BEEN 1 EMPTY SEAT. AFTER DETERMINING WHO THE EXTRA PAX WAS; IT WAS COMPUTED THAT I WAS 100 LBS OVER GROSS. I WAS OVER CONFIDENT IN THE RIG'S ABILITY TO MANAGE THEIR CREW CHANGE AND DID NOT PAY ATTN TO THE LOADING. I DID NOT DOUBLECHK THE REAR BEFORE TKOF. NEXT TIME; SHUT DOWN AND SUPERVISE THE LOADING INSTEAD OF DOING A 'HOT' LOAD; ESPECIALLY SINGLE PLT.