Ok, I got it, it was much easier than I anticipated it to be. I hope this helps somebody else along their way...
The add_action('user_register', 'jarredsUserCategory);
link calls a function that I've written, named jarredsUserCategory. The user_register hook returns a $user_id variable to be used, it's just not passed in the typical manner that variables are passed between functions (as far as how you know it's being passed). You still use it exactly the same in your own function.
So, for example, the jarredsUserCategory plugin file may look like:
function jarredsUserCategory($user_id) {
echo($user_id);
}
add_action ( 'user_register', 'jarredsUserCategory' );
So there it is, it's much more simple than I had anticipated it to be. WordPress continues to make me happy...
The function that you create can be anything you like, I'm going to try to use it to create categories for each individual user that signs up. Now actually naming the category properly so it's display name makes sense and looks right, and actually getting the user to only post to that category are completely different things, but 1 thing at a time, right?
Thanks again!