While it is fine in informal, first-name-basis settings, I would not address a group of, say, visiting female college professors as "you guys." If I were culturally an American Southerner, it would be okay to use "y'all" with this group, but since I'm not, I'd probably go with the same words, differently arranged: "all of you," as in "if all of you will please step this way...." I used to host historic walking tours and this worked fine for me, no one either lost or offended.
Of course, the real culprit here is the English language for lacking a clearly plural form of "you" and requiring an add-on word when the plural must be emphasized.