Congrats on making it to nationals (:
In most cases, you can achieve the same functionality of global variables without using them; many programmers usually discourage them since they take up memory and potentially make the program harder to maintain. That is usually only relevant for programs that are larger than a couple files, and for a small 90-line code it does not matter so much. In your case, the visitor and home arrays could have been declared in the main, and then printInnings() could have taken two array/vector arguments.
Your commenting is generally good; the idea behind commenting is to explain methodologies behind blocks of code (like functions and loops) or variable usage rather than what a single line does. So a good comment will explain, for example, the purpose of a function and how to use it, but a bad comment would attempt to explain what a line does when the code itself makes it clear. In your code, the comment on line 52 is of the former (although I would place it before the loop rather than on that particular line to make it clear that it describes the function and not the line), but the comment on line 20 is of the latter.
How you decide to treat the data in the file depends on how robust it needs to be. Using H instead of HR is perfectly fine if you're allowed to assume that your file will not be corrupt; you might notice that my code makes the same assumption (though I should have made a comment specifying that; shame on me ;)
Beyond that, there are a couple of pointers I could make. For example, the advanceRunners() function takes a vector argument and then returns a vector argument. On the computer memory, it is actually creating a copy of the vector you give it, doing stuff to that copy, then returning that copy. For large vectors, that can take quite a bit of time, but in C++ there are ways to accomplish the same thing without making copies at all. For that I would do some reading on passing by reference in C++.
Finally, in parse(), you have a repeated loop structure (loops that basically do the same thing) indicating that you could have used a function instead, but since you only have two and they're in the same place, it doesn't really matter. It begins to matter when the same code structure is used across multiple files or more than twice, as a single function is much easier to modify than a number of loop structures in different places.
Good luck at nationals!