I posted a reply to another thread similar to this, so I will cut and paste here, with some modifications.
Unless you have raised him since the moment he popped out of his mom, you just don’t know how he was treated until he got to you. Maybe he wasn’t socialized with people as well as he should have been. If not socilaized at a young age and with continous socialization throughout his life, issues could arise. The key is stopping them before they get out of hand.
If you are expecting people over, put your dog on a leash so that YOU have control over her behavior. You want her to associate people who are coming over with a POSIVITVE thing, and not a negative thing Teach the dog an alternative behavior, such as sit, when you open the door. (The dog already needs to know alternative behaviors before you attempt to utilize them.) If someone else can have her on a leash away from the door, that would be helpful too in the beginning.
If he doesn’t run away and just stands there quivering, then you don’t need a leash. It is all based upon how you react and how other people react. Some dogs don’t like people looking at them right away. Let the dog go and greet the people, rather than the people greet him. Allow him to get comfortable on his terms.
One way to start working on this is to set up fake meetings. Get some people together and have them one by one come over and go through the normal routine. Make sure they have yummy high value treats to give. When they come in the door, just have them ignore your dog, and toss some treats down in his direction (that way there is no hands close to her mouth), and walk inside. You may need to do this over and over until he understands that people coming over=YUMMY treats. This may take several days, weeks, and months. You just have to be patient. Once people are in and your dog feels comfortable, release him, and THEN they can greet him and he will interact. Slow movements by the visitors are a plus too!
Have the people wear different things (long coats, ball hats, sunglasses, etc.) so that he gets used to people looking “odd”. Always utilize treats to the best of your ability. If he isn’t food driven, then use a favorite toy. Have these items OUTSIDE the door when people come over so they are already to go.
Each time he doesn’t show fear, praise and treat her like there is no tomorrow!!! He will then put two and two together. People coming over=treats=praise from my mom. He will eventaully associate people coming over as a positive thing, and that is what you want. You may need to continue doing this for the rest of his life, but you will be able to minimize how often you provide treats. He will perform in anticipation of treats.
You need to train a dog positively, or you won’t get much out of your dog. Be gentle, patient, and consistent. Those are keys. You need to be relaxed and calm. Dogs can read us better than we will ever be able to read them. If you show tension, your dog will know it, and will react accordingly. Just be as calm as you can and work through the exercises often. There may be set backs, but just know that they can and will occur. You just start back from sqaure one.
Hope this helps!