I find that there is a ton of mis-information floating around gun shops.
Adding a foregrip to a shotgun is perfectly legal. The only circumstance requiring special registration would be when adding a foregrip to a pistol. This includes handguns, AR/AK pistols, etc. Adding a foregrip to a pistol would constitute the manufacture of an A.O.W. and is illegal without first receiving an approved ATF Form 1 and the required $200 tax stamp.
There is often confusion centered around the "cruiser" shotgun configuration (pistol grip only). Some will say that since it has never had a stock affixed to the receiver a cruiser is not a shotgun, but rather technically a "pistol" and in that case could not be fitted with a forward grip without AOW registration. This is only half correct. While the cruiser configuration is technically not a "shotgun" by definition it cannot be defined as a pistol either. By definition a pistol is characterized as designed to be fired by one hand and has a RIFLED barrel not to exceed .50 caliber. A 12 gauge smooth bore is roughly .70 caliber which puts the cruiser well outside the legal definition of a pistol. As long as the overall length is greater than 26" it cannot be defined as an AOW either. Technically speaking, a 12 gauge manufactured in a pistol grip only configuration with an overall length greater than 26" is undefined according to NFA regulation.
In defense of the confused, it's easy to see how they got that way...