You crate or pen him every time a guest prepares to leave so that he can't bite someone milling around the entrance.
You give him a treat if he does not bark.

Then you can try to have him on a leash while you say goodbye and you treat him constantly if he doesn't bark, and doesn't get excited.

Then you try the same thing without a leash.

Yes, you should consult a behaviorist/trainer. They will be able to observe your dog's behavior/reactions and give you useful strategies for managing/modifying his behavior. Even thought he is a small dog, it is a pretty serious issue that he has bitten your guest, it is unlikely that the bite was accidental and if you don't address the issue, it will probably happen again.

My shihtzu/maltase (3 years old) dog has been barking at my guest when they leave my house. It started one year ago but we never pay attention until recently. He accidentally bite my friend last week when he was about to leave my house. The wound is not deep and didnt go underneath the skin. But now I am concern. Any idea when?

We have a 1.5 years old toddler in the house and expecting another one coming. But at home, my dog is fine just when guests leave the house, he will go crazy barking and even chase them outside the door.

Should I find a behavior specialist?

Thanks.