Much as I try to talk the talk, I am not a dog training professional. If your dog is truly displaying aggression, be it human aggression or dog aggression, you should seek help from a Board Certified Veterinary Behaviorist, who will have studied extensively in areas such as those to meet your needs. The Internet, while a magical place of information and fun, won't necessarily be tailored to your dog and your dog's needs, or adequately explain how you yourself can "fix" things, or even give an accurate and appropriate diagnosis.
That said, I covered briefly what your dog can do when you have friends over. I've posted a picture (that I'll post again here in a few words) of Elka waiting for a delivery person. But, what do you do when a stranger comes to your house? Jehova's Witness, repo man, delivery personnel, door to door sales? What is appropriate for your dog to do, and what do you want your dog to do?