The use of "guys" in this sense is very informal, and usually used when addressing a group of people that you know well. In this context, it means the same as "everyone." So, yes, you could say it to women also, no one should be offended. And yes, I've heard women say it to their friends (all women) when they are leaving. I would not use it in a professional setting or with to address people that you hardly know. Hope this helps! J.