DHC8 CAPT ALLOWED A PLT FROM ANOTHER COMPANY TO OCCUPY THE COCKPIT JUMP SEAT CONTRARY TO NEW COMPANY SECURITY PROCS.

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APPROX 5 MINS PRIOR TO DEP; A PREVIOUSLY EMPLOYED COMPANY PLT REQUESTED TO RIDE THE JUMP SEAT. THE PLT IS CURRENTLY EMPLOYED AT ANOTHER ACR. AFTER CHATTING A MIN; THE PLT ASKED IF HE COULD RIDE UP FRONT WITH US. I INFORMED HIM THAT WITH OUR NEW SECURITY PROCS; IF A SEAT WAS AVAILABLE IN THE BACK; HE HAD TO SIT THERE. WE HAD 35 PAX (ACFT HOLDS 37) SO WE HAD HIM SIT IN THE BACK. JUST BEFORE DEP; THE GATE AGENT ASKED IF WE COULD TAKE 2 MORE PAX; I SAID OF COURSE. I THEN HAD THE JUMP SEAT PLT COME UP FRONT TO THE JUMP SEAT. WE GAVE THE PLT THE JUMP SEAT BRIEFING AND THEN BEGAN WORKING ON THE WT AND BAL PAPERWORK. WITH A FULL LOAD OF PAX AND BAGS; IT TOOK US A FEW EXTRA MINS TO ENSURE WE WERE UNDER OUR MAX TKOF WT. WE THEN CLOSED OUT OUR FLT AND PROCEEDED TO DEST. ON THE NEXT NIGHT ON THE SAME FLT; WE WERE CLOSE TO OUR MAX TKOF WT. AS WE WORKED AT WAYS TO ENSURE WE WERE BELOW MAX TKOF WT; A PLT IN THE BACK OFFERED; TO THE FLT ATTENDANT; TO RIDE THE JUMP SEAT. THE FO AND I EXPLAINED THAT THE PROB WAS WT AND SO IT WOULDN'T HELP. ADDITIONALLY; I EXPLAINED WE DID NOT HAVE A JUMP SEAT AGREEMENT WITH THE PLT'S COMPANY. THE FO THEN SAID WITH OUR NEW SECURITY PROCS; A NON COMPANY PLT COULD NOT RIDE IN THE JUMP SEAT. WE THEN REALIZED THAT THE PREVIOUS NIGHT THAT WE SHOULD NOT HAVE LET THE OTHER PLT RIDE IN THE JUMP SEAT. I DID NOT MAKE THE CONNECTION THAT WE COULDN'T LET HIM RIDE UP FRONT. ALTHOUGH I HAD PERSONAL KNOWLEDGE OF THE PLT AND THUS I DON'T FEEL SECURITY WAS JEOPARDIZED; I REALIZE I DID NOT COMPLY WITH OUR NEW COMPANY PROC.