It sounds to me like she might be a bit nervous around new people since she's good with people she knows
and this nervousness comes out as aggression
What you can do is borrow a good friend (Bribe them with lunch or something)
have them come over and tell them to ignore the dog
also give them a handful of treats
when they come in and the dog goes to bark
just tell your friend to indiscriminately start dropping treats every once in a while
this will occupy her and she will eventually get the idea that strangers aren't something to be afraid of
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