You should probably find a good, local positive based trainer to help you with this. It sounds like she did not have enough good, early socialization. That doesn't mean you can't improve things now, but it will be more work.
For starters, people who come into your house need to be told ahead of time not to even LOOK at her, and under no circumstances should they try to touch her right now. It will only scare her worse. For first steps, you could give each willing participant a handful of small treats. Have them sit down with you and converse, totally ignoring her. If she moseys around, (she probably will) have them gently toss the cookies out to her, one at a time. They can talk to her gently as they do this, but they still shouldn't touch her.
Over time, as she shows a more positive attitude toward visitors, you can EVENTUALLY let people offer their hand, but ONLY when she comes over to make friends first. If she shows any sign of hesitance, you are moving too quickly. She may always be a dog who is "slow to warm up", and may not ever feel comfortable being handled by strangers, but you can certainly get her to a place that she is not stressed out by visitors.
BTW, alarm barking when people first come to the door is TOTALLY normal "dog behavior"... they are just telling you someone is there. But you eventually want her to understand that you've got things under control. Kodi barks when someone comes to the door, but as soon as I let the person in, and tell him it's OK, he stops. He does this even though he's friendly with EVERYONE one they're in the house... I swear, he'd show a robber where the silver is!