Can "guys" refer to girls and women?
I've seen people starting a speech with "hey guys" in front of a group of females.
But as far as I can remember, there was once an Australian clarifying that guys can only be used to address men.
Can anyone tell me what your opinion on this is?
Thanks in advance.

Question

Can "guys" refer to girls and women?
I've seen people starting a speech with  "hey guys"  in front of a group of females. 
But as far as I can remember, there was once an Australian clarifying that guys can only be used to address men.
Can anyone tell me what your opinion on this is?
Thanks in advance.

Answer

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.

Answer

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?"

Answer

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