What I didn’t realize at the time is that there is no context in which ‘guys’ is unambigously non-gendered.
The End of Guys
Jack Danger

Julia Evans did a great survey on the use of “guy” or “guys” and the contexts it could be used, and how people (broken down by men & women, no “other” option alas) perceive the gender-neutrality of the term in each context. You can read it here, I highly recommend it.