I absolutely agree with Aly on this one. Many times if you have an aggressive dog - shock collars, citronella collars, prong collars, etc. can backfire and make them more aggressive. It is kind of hard to judge through cyberspace, but there are so many reasons a dog could become aggressive, that a vet visit (for possible medical issues) and then a behaviorist is the best way to go.

In the meantime, I would crate the dog when you have company over. Take him out of the "pack" if he is not going to be polite to everyone.

Good Luck