I don't think its necessarily a sign of aggression. Gunther used to bark at everyone and anything outside our yard. I've gotten him to the point where he is allowed to bark a few times, but not exessively or obsessively. If I tell him to knock it off, he'd better knock it off! He's now started doing this at the dog park, to people outside the park, and even people inside if he didn't notice they were there and they moved or surprised him.
I think for some dogs, it has to do with protection, or just plain notification. I think if you can teach her a key word like "quiet" or "no bark" it can help tremendously. Gunther knows "quiet", but sometimes he still needed the added emphasis of a squirt bottle.
I would still try desensitizing her if at all possible. Get friends to come by that she doens't know yet or family dressed up in strange clothes. Especially if she is scaring the other people. For some dogs, this just eggs them on and makes them want to do it more, b/c they are getting a reaction out of the people, and you certainly don't want that.
