We have the same with our two hounds but we are now training them to "sit" whenever we tell them to. If you do this when visitors arrive and only give them treats when they obey you should be able to solve this gradually. If you can get your visitors to command them to sit as well it should dawn on them that they only get nice things if they sit down and behave.

Also, do you put his lead on him before answering the door - that might give you a bit more control

This will take a while because it is a hard habit to break from when they were little and cute!