YOU CAN SAY "HEY , GUYS" TO A GROUP OF GIRLS; YES; "GUYS" IS PLURAL FOR "GUY"; YES; GOING OFF THIS PREMISE, IT'D MAKE SENSE ONE CAN SAY "THAT GUY" TO REFER TO A GIRL; THAT MAKE SENSE TO ME; "SEE THAT GUY OVER THERE? I HAD SEEN HER BEFORE."; ACTUALLY, NO, GUYS IS ONLY GENDER NEUTRAL IN PLURAL FORM