Territorial Aggression

Question
Hello -
I have a wonderful 11 month old fixed beagle mix named Maggie. She is sweet and loving, and I went out of my way to make sure she was fully socialized as a puppy. She is rather high energy, so she is sent to Doggy day care at Camp Bow Wow 3 times a week, and stays at my mom's house two days a week where she can play in the yard with my parents golden retriever while I am at work. She is a regular at dog parks and we have traveled with her extensively, with no issues. She also went to and completed puppy kindergarten (an introductory obedience class) She is pretty strong willed, but we have mastered sit, down, leave it and are still working on come.

Maggie in the last few months, particularly at my mom's house has begun barking a lot at passersby, and has shown territorial aggression to 4 select people. We didn't understand why exactly she disliked these 4 because aside from the cable man, I can tell you that all are sweet kind and animal people. Last week, a friend of my mom's came over, Maggie showed aggression and then bit her in the back of the calf. Thankfully she was very gracious about it. We brought in a trainer that was recommended by our vet. He felt that we needed to get her back into obedience school and get her to the point where we can do a down stay for 30 minutes. We have enrolled her and she starts next week. She shows no aggression in places that aren't her own. Although I am terrified that this behavior is going to spread. I was slightly unsatisfied with the trainers answer since I believe her aggression is fear based, I'm not sure if I would be treating the problem by doing that alone.

Any help or guidance would be greatly appreciated. I love my dog, and want to do the right things so that we have the longest happiest life together possible.

Thank you,

Erin White

Answer
With any form of aggression, I am leery of any trainer who uses physical correction or punishment.  I wanted to tell you that up front, as the dog only needs training and management, not "discipline" that could make her even more anxious or frightened (about 80% of aggression is rooted in fear, not "dominance")

The first thing you want to do is, in a non-threatening way, change your dog's emotional response to strangers.  Barriers increase territorial aggression, so don't leave her outside to run the fence line, and limit her access to windows and doors that she can see out of.  If you think about it, her strategy of barking, lunging or nipping is working - for her.  That's because the people do go away (she thinks she was the reason, regardless if the meter reader or jogger was going on to the next house anyway.  Dogs repeat behavior that works.
The other thing you can do is to classically condition your dog to believe that strangers are good.  Play the "open bar, closed bar" game.  When you are with her, and a new person appears, you feed her until they leave.  If she is too aroused to eat, you must increase the distance between you and the scary person until she does feel comfy taking a small treat.  Pay no attention to any bad behavior, just keep feeding until the person is out of sight, then stop.  You are teaching your dog that when strangers appear, good things happen for her, not bad things.  
You can also counter-condition your dog to do a behavior on cue that is incompatible with chasing, lunging, barking & nipping.  The "Go to your mat - with down/stay" is good for that, but be careful to teach it in a positive manner.

Here's a short article on fear aggression with territorial aggression toward people.  It's by Nicholas Dodman, who is a veterinarian behaviorist, and one of the top people in the field of canine aggression.
Here's a video on training the mat exercise using clicker training:
http://www.youtube.com/watch?v=tVGaslyGaGE