As a Delta Red (NW merger transition period) we were encouraged to non-rev on DL to familiarize ourselves with our new colleagues. We're listed behind DL proper but before everyone else. There were 4 seats left- 2 for me and another NW, and 2 for 2 other guys from another airline. When I approached my seat, they double booked my seat assignment. The other two OA employees took their seats and quickly settled in, of course, seeing our predicament. Rather than having the lower priority guys leave, they made me and the other NW pax leave. We went with the flow. Absolutely no reason to make a big fuss. We know how this works. I had to take NW via MSP to get to ATL, and subsequently missed the flight to SJU and had to spend the night in Atlanta. It's all cool, but I was surpised that that the FA didn't make any effort to remove the last 2. I got a meek apology but that was it.