I don't know about the numbers, and no one can really say what they'd do for sure in most any situation until it's upon them. However, I know there are a lot of guns out there in a lot of people's hands who know how to use them relatively well, at the least. I don't think any sane person really wants to shoot someone or plans to, but, given a situation that calls for it and ultimately is between that person and their family's life and the life of someone who wishes them harm, I think most people (even ones who you think absolutely wouldn't) would pull that trigger without hesitation.
Now, there is a big difference between working as an organized group to fend off invaders at the country level and standing right next to your family to defend them. But at some point with an enemy army on the doorstep, those two scenarios start to become one and the same. The defenders almost always have the greater motivation to win because, if they lose, they lose everything.
I also think that one motivated person with a bolt action hunting rifle would be a lot more effective against invaders than two of your typical AR 15 toting tacticool "range soldiers." That hunter has some experience in the woods, has used his rifle in all weather conditions (usually in less than ideal conditions) and knows what it's like to look through the scope at a living creature before taking its life. I don't want the last part of that sentence to be taken wrongly. There is a huge difference between taking the life of an animal and taking the life of a person (never have, hope I never do), and, for the record, I don't think hunting leads to killing people (more than likely the opposite, in my opinion). However, there is a big difference in hunting and punching paper in beautiful weather.
Even if hunters don't organize at the national level but almost every house or neighborhood is reasonably defended, invaders would still have a tough time. Especially, if our actual military put up a good initial front.