I've read through a lot of these, and I'm not a professional but I have a dog that was vary wary of men and growled at all of them (still does occasionally), but now is mostly fine with them, and best friends with the ones she does growl at after only a couple of minutes. I achieved this by having one guy that I know and trust and that loved dogs and wanted to help my Jessie coming over and sitting on the floor and ignoring her until she came up to him and wanted to interact. This was possible mostly because she would keep away from them while growling and just being vary nervous. By him sitting on the floor and watching tv or talking to me he didn't seem so big and threatening so she would eventually figure out that he wasn't a threat and got curious about him... mind you he pretty much stayed still and if he did move it was vary slowly so not to spook her... After a while he was able to just come over and move around normally and she eventually got happy to see him... after she was won over to him it was easier to do with my next guy friend and a quicker process... now she's just fine, or if she does get uneasy all I have to do is look at her and tell her that it's ok in a firm but kind voice and she'll either go lay down or ask to be pet... I realize that different dogs need different things, and maybe this isn't the best idea for you, but the principle is there, start slow, and with only one new person exposed to your dog at a time until she get's used to that one visitor and go from there, make sure it's someone you trust and that trust's you and that will listen... sounds like she needs to know that it's ok that other people come over and that it's even a good enjoyable thing. treats work, they help build trust and a positive experience for the dog.

Good luck, and I agree start with a closed door, or a large baby gate that way the dog can feel safe in her own area and view the visitor at a distance, ignoring the dog if she barks or misbehaves, that'll teach her that nothing's going to happen when she barks, if she's secured you won't have to worry about her attacking because she misread the situation. once she settles behind a gate have the guest toss a couple treats over to her, repeat process until she's totally docile and wants to meet the new person.

Just throwing ideas out there...

It sounds like she's stressed and maybe a bit territorial... just keeping her locked up all the time will only further her un-socialization, and her staying un-socialized might only make matters worse... baby-steps is key...