Aggression problem

It's hard to offer advice or suggestions based on what you told us.  Most dog aggression is fear based.  She may have never been properly socialized. 

  Find a good dog behaviorist and get a consultation done in your home.  Kudos to you for adopting this dog and for being willing to find a solution.  There are usually no quick, easy fixes for behavior problems.  It takes time and work but you need someone to evaluate what is happening in order to advise you on how to deal with the problem.  Sometimes advice given by people who never see the dog can be way off base and make the problem worse. 

First - does she have a crate?  If she doesn't, is there a safe place where she can be confined for short periods?  A place that you can convince her is hers, somehwere she is safe, and she can go there when she is tired or stressed.  This is essentially what a crate is, and every dog should have such a place IMO.  This alone can make a difference.

Second - when guests come round, they should be fully briefed to IGNORE her completely.  Some people think they they are good with animals and try to "help", but their appraoch just makes the dog feel more threatened and makes the problem worse.

Third - when guests come round, put her on a lead before they come in and don't allow this behaviour to continue.  The moment it begins, calmly get up and walk away with her.  Leave her safely confined - somewhere like a crate is ideal, but being in there should be a pleasant experience for her, so don't get cross or impatient.  Come back to your guest and ignore any fuss she makes.  She can come out after a few minutes, provided she is not making a fuss.  Calmly eject her again if she repeats the behaviour, or alternatively, leave her confined with a safe chew  or toy until the guest leaves.  If she manages to relax and control herself, you could ask your guest to drop some tasty treats on the floor (bits of cheese or chicken or hot dog sausage, or liver brownie), to help change her perspective of people.  At first she might be too anxious to even consider food which is why the tastier and stinkier the better.

What you do when she reacts this way is important.  You have to stay calm and not get wound up.  It's hard, but having a plan of what to do when she does this can help clear your head and makes you act calmly and decisively.  Just this on its own has a positive knock on effect on the dog's behaviour... it's almost like they think, "it's OK, someone is taking charge".

Last but not least - GET A MUZZLE.  You haven't had her long, and you may not know everything about her history, temperament and thresholds - so BE SAFE.  Keep yourself safe, keep your geusts safe and keep HER safe.  TRAIN her to accept the muzzle using food treats.  You can poke a couple thru the end of a basket-type muzzle to encourage her to stick her nose in and work up to clipping it on and leaving it on for short periods.  Make muzzle-time = party-time.  You don't want her to think everytime the muzzle comes out something BAD happens, it will only impede her learning the new things you want her to learn.

I agree with everything Jackie said.  This sounds like a fearful dog and he would likely be better off put in his crate in another room, safely away from the "invaders".  At least for now.  Eventually, with,  professional help you can work on desensitizing him to people.  He hasn't been in your home very long, and goodness knows what the poor things background is, so give him a "safe" place when you have guests for now.  Almost sounds to me like too much new stuff too soon.

 You will probably not get a good handle on this dog's actual personality for a few months.  Right now, the poor thing probably feels like he has been dropped on Mars and the little green men are out to kill him.  That's how many dogs feel when they lose their social group and are suddenly placed in another.  He doesn't know you that well yet, although you are his only security, so don't do anything to destroy his trust at this point.  One thing you could do is to put up a gate, and YOU feed him tasty treats when guests appear in the room (they shouldn't interact with him right now), and stop feeding when they disappear.  If he doesn't take the treats from  you, he's probably too nervous and you should consult with a behavior professional who may be able to help you by supplying a larger space in which to work, and some "volunteer guests".