When I address more than one person, I always use "you guys," regardless of gender or the social context. When I refer to "guys" in the third person plural, I am always referring to males.

Yes, you can use it to refer to a group of men and women, you can even use it to refer to a group of all women (although it would probably be better to say "girls" in that case). It's because modern English lacks a plural "you". So we actually use the phrase "you all" (in southern USA, "y'all"), or "you guys" as a pronoun, functionally! "You all" is acceptable in formal situations, "you guys" is informal and friendly. And 'guys' can always be used, even if there is not one man in the group! "How you girls doing?" sounds a bit strange to me.... but it makes sense... hmmm. I still think I would say "How you guys doing" if i addressed a group of all females. Or maybe 'how are you all doing' to avoid saying 'guys'. but 'how you guys doing' is much more comfortable to say, we use it a lot. In this particular case, when we say 'guy' it means more like 'a person' than 'a male'.