Again, depending on the teams (local or historically "big" rivalries will get more cops buzzing around to quell any trouble) obviously the security is going to be stepped up in and around the ground.

However, United and Crystal Palace are in no way shape or form major or big rivals with each other, so there would be no need than anything but the regularly mandated PL guidline number of Officers. And again, whatever cordons and strict security measures may have been in place before the game are probably going to be relaxed to expedite the timely removal of the people from the area after the game. All they had to do at that point is say they're supposed to meet someone at the game, is it alright if they stand a bit further up to make themselves more visible to their mates and they can pick their spot.

(Any time I was near Old Trafford, I was inside watching a game, but I've gone to concerts where an entire town was shut off by the cops from about 10 am, yet at 10 pm when it finished, I was picked up by a friend's mum inside the police cordon, not really a stretch to imagine it happening outside Old Trafford.)