You are not alone. Most dogs bark some when there is a surprise visitor at the door. Here are some of the reasons they bark and what you might be able to do to help them stop barking sooner.
The dog doesn’t know what to do when people come. 
One of the best things to do is to teach your dog a routine that will happen when people come to the door. Decide what you want your dog to do? Do you want them to stand next to you, go to their bed, go behind a baby gate or into a kennel, run and grab a toy? We are teaching the dog an alternative behavior. Which solution will you choose? It depends on the dog and their personality. Choose a routine that is most suited to your dog and your household. Teach them this routine with people in your family. Practice it repeatedly until your dog does it super well. Then add in some knocking and excitement.
The knock or doorbell startles the dog. Practice knocking very softly, then give the dog a treat. Do this 10-15 times in a row. Later do the same thing, but see if you can knock just a tiny bit louder. Continue doing this until when your dog hears a full knock, they look to you for a treat. What if you have a doorbell? You can’t exactly turn the ringer down. Try recording the doorbell and playing it very softly at first, and give your dog a treat. Continue as outlined with the knocking. Gradually make it louder. Once your dog is great with the recording, try it with the real doorbell.
The knock or doorbell predicts someone scary. Is your dog afraid of strangers who come into your home? A very simple solution is to teach your dog to run into their crate or behind a baby gate if the doorbell rings. Give them a bone or stuffed Kong to keep them busy while you answer the door. This gives your dog something else to do so they don’t have to interact with the scary person. Practice so your dog is a pro at doing this before someone comes over. Be sure to get help for your dog so that they can become more comfortable with strangers.
Can’t you just ask them to be quiet and stop barking? You can teach your dog a quiet cue. However, you will likely need to follow up with another behavior (such as go to their bed or grab a toy) because when they stop barking, they won’t know what else to do (see the first bullet above). How do you teach a quiet cue? The first thing you do is to say your word, “Quiet” and then give your pup a treat. Repeat this 10 times in a row, 3-4 times over the next day or two. Then practice by knocking on the door, when your dog barks, say “Quiet” and treat when he stops (he will stop because he has learned that the word predicts a treat, so he will start to look for that treat). Repeat 10 times in a row, 3-4 times over two or three days. Then give it a try when he barks for real. Be sure to reward generously every time.
We often need to use a combination of solutions to get the best results. And it is always important to remember that we most likely won’t eliminate the barking altogether, but rather reduce the intensity and have the dog stop barking sooner!
