While it is relatively come usage to use "guys" to mean a group of people of any gender, it is also quite common for women to not like to be referred to as guys. I have seen women get offended many times when they were part of a group referred to as "guys". Same thing goes for "dude", which I have had to retrain myself on - I grew up in a stoner/skater generation that used the word "dude" as kind of an all purpose interjection, but many women don't like to be referred to as "dude". So I suppose it all depends on if you care at all if you unknowingly are going to offend a woman or women you are communicating with. "people" is an easy swap out for "guys" if you want an alternative that isn't going to offend anyone unless you happen to have some extra-terrestrials in your target group ;)