That's sort of what the national guard is for. They train a couple weeks per year and respond to situations like this.
I am thinking more like a level below volunteer firepeople, with some regional and national coordination.
Or form your own. visit your local shooting range for details.
Like, ah, these fine gentlemen. [1]
[1] https://www.vice.com/en_us/article/new9wd/the-birth-of-canad...
I'm thinking of more of a social graph where civilians who are going to help in these sorts of situations can be organized more efficiently. For instance, instead of having a bunch of people drive their personal cars out there, going out as a group would improve the individual's abilities to help efficiently.
They will also be deployed domestically to repress public uprisings, etc.
If you want to help people, you're better off becoming a firefighter. Unfortunately, outside of some small community volunteer firefighter groups, this really isn't an option. The National Guard does not fill this gap.