Visitors

First.
Remember to never ever punish the dog for her fear/aggression towards strangers.  No yelling, startling, leash yanking, hitting, etc..

Second
As the benevolent leader, your are responsible for your dog.  If anyone tries to touch your dog when she is upset, step in front, block that person from touching your dog. If the person doesn’t want to listen, then either remove your dog (no snatching or yanking) or politely ask the visitor to leave.




De-stress
Since your dog has already formed a negative association with visitors, I suggest that you and your roommate come to an agreement that you not have any guests over for 1 week. This will give your dog a little time to de-stress while you get to work on a non aversive way to help her with her issues.
The Book Scardey Dog more thoroughly explains how these stress hormones can build up and why it’s important to allow a de-stress period.

Management
Management.  Since you know your dog is going to get upset about visitors, don’t have her out when visitors arrive. Ask your visitors to call from their cells as they pull up to your house/apartment.  That way:
1. This will give you time to put your dog into her crate or another room and she won’t associate the crate with a bad thing - visitors. Not at first anyway
2. The doorbell won’t get her all revved up

Teach an alternate behavior
I would start with just the doorbell and teach the dog an alternative behavior.  Teach your dog to “go to place” or “go fetch” or “go to crate” or “go get your toy, ” or “ sit.”  Then put it on cue with the door bell. You don’t want the dog to see YOU ring the doorbell.  Your roommate or another friend can help with this. Remember to teach any and all commands using positive reinforcement only. Using aversive methods to train then asking for those commands can invoke negative feels towards the strangers.
No yanking, yelling, pushing, etc.. Actually, don’t use your hands at all.

Desensitization and Counter Conditioning
Get your dog used to people coming over through desensitization and counter conditioning.  Always start below threshold and make it fun. If at anytime, the dog gets upset, then you have moved too fast and the exercise will be counter productive.
To start below threshold, the first visitor will be the roommate. Have your roommate pretend to be a visitor and go through your routine - like “go get your toy.” Treat (really really good treats like baked chicken) the dog or have the roommate give the dog a sit and treat the dog.  Always provide some type of alternate behavior so the dog won’t feel like she has to resort to barking and growling.

After this works well with the roommate, try a calm friend whom the dog is comfortable with. Stay below threshold and keep doing it until the dog is happy.  This could take minutes, days, weeks.

Then with a friend who is a little less known but who isn’t upsetting.  Once you move to unknown people, for safety’s sake, you at the pet parent should be giving the treat instead of having the guests do it.
More info in References below on how to continue to do this.

If you have guests who are unwilling to help you with this, don’t force them  into it. Ask politely, if they don’t want to help, then put the dog away. If they refuse to follow your instructions then put the dog away or ask them to leave.

Other People
Don’t allow anyone to:
Pet your dog on the head
Touch your dog when she isn’t ready to be touched
Pet your dog roughly
Wrestle with your dog
Encourage jumping up by patting their chest, etc..

Don’t allow anyone to reward undesirable behaviors like jumping up, barking, etc..

Positive Reinforcement Works
Some people might tell you that positive reinforcement doesn’t work.  
Behaviorist and author Jean Donaldson says that people can’t execute a method incorrectly then blame the method. Positive Reinforcement when executed correctly does work.  Negative reinforcement  might work with some dogs but:
1. It hurts your relationship with your dog
2. It doesn’t always work and in many cases it will make your dog much much worse.
You don’t have to take my word for it. Just read the works of people who have studied animal behavior, who have advanced degrees in animal behavior, who do seminars all over the world.  Remember that TV is for entertainment. Books are for education.
Everything isn’t about dominance

Behaviorist and author Ian Dunbar says that suppressing a growl is like removing a ticker from a time bomb. So “correcting” your dog when she barks or growls just teaches her to not give a warning next time. And she could bite without warning

Furthermore
Are you working on ways to keep your dog calm yet stimulated and exercised? Does she go on long walks every day? Get training lessons? Do you reward calm behaviors through out the day?

Calming Remedies
Check with your vet on using some things that might help your dog be more relaxed
Bach Flower essences
Tryptophan
Valerian Root
Dog Appeasing Pheromone

...Just in case something does happen, make sure you have shot records on hand and easy to find at all times.

References

Barking: The Sound of a Language by Turid Rugaas. Start here. It’s a small book and can be read in one setting

Patricia McConnell - “The Other End of the Leash” - has some good info on door greetings and lots of other great info. Read this one next.

Patricia McConnell - How to be the Leader of the Pack and Have your Dog Love you for it.

Patricia McConnel - Family Friendly Dog Training

Patricia McConnel - Cautious Canine

Ali Brown - Scardey Dog - this has info on dogs who are scared of other dogs but the principles can be applied

Leadership and Dominance: http://www.stubbypuddin.com/2011/02/on-leadership-and-dominance-years-ago-i.html

Benevolent Leadership: http://www.stubbypuddin.com/2011/02/benevolent-leadership.html

Managing Behaviors: http://www.youtube.com/watch?v=el1h90bq998

How do I get my dog to like other dogs:  This is for dogs but it has good info on counter conditioning and desensitization.  Just replace “dogs” with people. http://www.stubbypuddin.com/2011/04/how-do-i-get-my-dog-to-like-other-dogs.html

Kitchen nightmares - More good info on CC&D: http://www.youtube.com/watch?v=3-CCJxF-9U4

Teach your dog to sit: A hands off - non aversive way to teach your dog to sit: http://www.youtube.com/watch?v=faqSuJFuxGg

Reward Calm Behaviors: http://www.youtube.com/peteducation#p/u/12/EIvWIyVZoGM

Things Humans Inadvertently teach their pets: http://www.youtube.com/peteducation#p/u/10/EQpP02nTVeg

Stopping Unwanted behavior before it starts: http://www.youtube.com/peteducation#p/u/9/YoHTir_uK1o

Inside looking out: http://kpk9listen.blogspot.com/2010/10/inside-looking-out.html