I saw a similar situation on this dog show with Ceasar Millan called the dog whisperer. If you haven't seen it watch it or go to his web site because he is really good! Anyways, What he explained is that when your dog is doing this he is being protective and claiming the door as his. He is also being protective of the house and telling who ever is there this is my house so beware. What Ceasar did was "claim" the door as his. Whent the dog would go up to the door barking he would move the dog away from the door and make him sit and stay and every time he would bark say NO or SSHH in a loud/firm voice. If the dog would move he would take him back to the spot and make him sit and stay again. The dog was not allowed to come and 'greet' the person untill the owner said so. This process has to be about focusing on the dogs behaviour and correcting it right away. If you don't correct right away then they get away with the behaviour and learn it is acceptable and that you ( the owner) can't do anythig to him. You could try this.
I had a similar situation with my dog where she would bark at people through the window or if she was outside and to change it I corrected her right away. If I heard her barking I would say in a loud firm voice NO. If she continued I would remove her form the situation, ei, close the curtains so she can't see the people outside or bring her back inside. Also, I would use a shaker (water bottle with pebbles inside). You shake it when you hear them barking and it ditracts them and takes their minds away from the barking. Plus, it can be loud and I know my dog didn't like it.
Just some suggestions that might work
