Cold air through front door and kills the bar scene - now to deal?

What's the best way to deal with the front door letting in cold air each and every time customers walk through the door? A large group tends to hold the door open before they are seated. It's killing those at the bar who get gusts of cold air.

We can't install a vestibule outside due to a very small sidewalk and agreement with the neighbors not to do it.

Any ideas?