In the United States, you will hear "guys" for any group of men or women all the time. You will even hear some women call other women "dude". "Guys" works for an all-woman group as well.

It depends on the context. If you are telling your friends about something, for example "we were working on...and a bunch of guys came to help...", that would mean men only, else you would say "a bunch of guys and a few girls...". When you address your friends in conversation, most would use "guys" to refer to both men and women. For example "where did you guys go yesterday?"

Most females will not be offended by anyone making an open statement such as "hey guys." Sometimes, it is used as a conversation starter.