PIB (pronounced "pibb") is an acronym for "Person In Black". This is a generic term applied to presumably depressed* teenagers who wear a lot of black clothing, including Mansonites (In this case, Marilyn Manson fans and not "followers" of Charles Manson), Punks, Goths, and the like.
PIBs can most commonly be found in late-night coffeeshops, on streetcorners, and in other traditional places of social gathering. They are typically surly and unfriendly but are not considered dangerous as they fear the light and can be repulsed with the aid of a simple flashlight, or in the case of larger numbers, a nautical strobe.
* It is not safe to assume that these people are depressed, but this is the basic cultural assumption.