The term guy is generally restricted to males, as in Was that a guy or a girl?, but the form you guys may be used for groups of any combination of genders whether it is all male, all female or any combination.
Unlike y'all or the French vous, you guys is typically said only once in a short conversation, thereafter you is used. For example:
I'm going to be on the outside basketball court at noon if you guys want to practice. You should bring some water because it's going to be hot.