I almost got stuck there a few months ago headed back to EWR. The loads looked great but the gate agent said they could only take 2 out of 10 non revs because of the cargo. I was last on the list with only one seat left but I got on because everyone else was in groups and didn't want to split up. I talked to the Capt on board and he squeezed out two more seats from load planning, so he told me to sprint to the back and hide so they wouldn't kick me off. A couple of the longest minutes of my life later and they closed the door. Good luck!