One should never try to approach a strange dog by petting them (or kissing) them on top of their head. The dog cannot see what the person is doing on top of their head and some dogs feel threatened by this approach. Remember, dogs have a much better sense of things then we do. When the dog is with a lot of people they don't know it is unsettling enough, throw in inebriated people and it's worse.
The proper way to approach a strange dog is with an open hand, palm up and from under the dogs head. Let them give your hand a smell and take a few minutes to see if the dog is open to being petted. If not, back off. A growl or bark is the dogs way of saying stay away.
Lastly, if the dog is uncomfortable about groups of people and the possibilities of someone getting snapped at or worse, bit, put the dog in another room closed from people getting to it. If someone does get bit I can pretty much guaranty that it is going to be attributed to it being a "pitbull" .