I had a very territorial and protective (but very controlled) dog (he would give little nips and "mouth" people but never full-on bite, despite having the teeth to do so
), and what my partner and I did when we first started dating was meet on "neutral" ground (ie: outside of the house, off the property), taking the furbaby for walks together (he just LOVED walks and was almost always too distracted by all the smells to be bothered with barking at or warning anyone away), and, yes, TREATS. It took awhile for him to acclimate to my partner, but it was absolutely worth it; he really came to view him as part of the "pack". (I have to throw a word of caution in about the treats and just say to be careful with them as sometimes they can reinforce the negative behavior and have the dogs thinking they are being REWARDED for what they are doing.)
So, for your most frequent visitors, see if they would be willing to put in the little bit of time and effort to do that (meeting off-property if it is only in the house the dogs are aggressive, finding something they really like to do and having your visitors participate in it, treats from visitors when the dogs refrain from biting/being aggressive).