For example, the sentence is "If you want to introduce someone into our company, you need to talk to HR "people" or "folks" or "guys""? Which ones are correct? If more than one, which one is most common and natural?
This depends a lot on who you're talking to and in what setting. Both "folks" and "guys" are probably on the too colloquial side. HR people is the best of the lot, but still not terribly formal. Something along the lines of "If you want to introduce somebody into our company, you need to speak to someone in the HR department," would probably be the most acceptable in a professional setting IMO.
In the US, it's common to say simply "You will need to see HR about that."