I would suggest a multifaceted approach:
*Teach dog a cue for quiet.
*Teach dogs that visitors are good - have visitors and yourself click and treat quiet (even a pause in barking to take a breath!) and treat.
*Have visitors be very boring except for treat dispensers - have them sit down, not doing much. Ensure they ignore the dog.
*If visitors are unwilling to help you overcome this, remove dog and put him in a different room.
If you need help or explaination on 'how' for any of these steps, let me know.