Apparently, many New Yorkers are unaware of the fact that if they hold the door open, cold air will blow in and warm air will blow out.
Actually, it's worse than that. See, heat rises. So the warm air inside the door rushes with gale force out and up into the sky, to be replaced at floor level by the heavier cold air, which in turn pushes what's left of the warm inside air up ... and out the door.
So when you (whoever you are) stand there holding the door wiiiiiiide open, waiting for your entire posse, or perhaps family, still ten feet away, to approach, going in or out, you're letting the warm indoor air out and replacing it with frigid air from outside.
And, in all likelihood, freezing a security guard or a doorman or the employees of a building, and perhaps also its patrons and/or inhabitants.
And a thermostat is triggered, and somewhere in the subterranean depths a furnace turns itself back on and burns a couple gallons of oil to heat the frigid air you just let in.
So for the love of God, or the environment, or even just your fellow human being ... please, please, shut that door.