"You guys" is a familiar, all-inclusive way of addressing a group of men or women directly. That said, there are some important distinctions you must understand.
"You guys" is more likely to be said in women => women or men => men or women => men or mixed-group => mixed-group contexts. It is less likely to be used in men => women contexts, but is still heard and would probably not occasion any confusion or merriment.
The singular "guy" is another animal. It refers to males. It is also used to draw gender distinctions in a general way.
A guy walked into my store and asked for some cigarettes.
There is no doubt that this is a man we're talking about.
In most plural usages that are not directly addressing a group, this rule also applies.
Guys are pretty simple, when you get right down to it.
This will also be understood to refer to men only. If you wanted to make the same statement about women, you would use another noun: women, gals, whatever.
However, you could say something like
My friends in San Francisco? Those guys are so crazy!
Now we're not sure we're talking about men. If the speaker is female, it might mean a group of women. Note that I say might. It is more likely she would still be talking about men or a mixed group, but you never know.
It's a hard word to pin down. Much depends on context.
On a walk yesterday evening I encountered a woman walking three dogs. When we got close the dogs started barking at me. The woman and I exchanged greetings, and then she admonished her dogs by saying: "Cut it out, you guys!" One dog kept barking, and she said: "Sally, stop it!" What to make of this? I suppose that for some people "guys" can refer to groups of dogs as well as humans, and not just male ones. Ain't English fun?
New Evidence (2023)
A young woman scientist who does YouTube videos just referred to herself as "a dark-matter guy" in a video that criticizes string theory. This is another reinforcement of a trend that shows that, 12 years after I first wrote this answer, even beyond the usages I described, "guy" is taking on a genderless sense quite independent of its original use as a gendered expression. Language is always in motion.