@tpg2114's answer is the right one from a policy standpoint, but it's worth making an extended comment to observe that "guys," specifically, occupies a strange place at the intersection of formal vs. informal language and gender.
It's totally true that, at least in the US, "hey guys" or "you guys" is commonly used as a genderless plural address. I naturally use it when I'm speaking collectively to my different-gender children ("hey you guys, come to dinner") and my children use it with equal comfort when addressing same- or mixed-gender groups of their peers. But all of us seem to use "guys" with a genderless meaning as a term of address, but with a gendered meaning when referring to an unknown person. This is especially clear among the teenage girls in my life, for whom a sentence like "oh my gosh, you guys, let's talk about guys" (using an all-female and an all-male meaning right next to each other) would be completely idiomatic and immediately understood.
That's ... a weird usage for a word, which I personally hadn't considered until an example like that one was made for me. Furthermore, for many older US-English speakers I know, the genderless address does not feel natural, and they feel compelled to address mixed-gender groups as "guys and girls" or the like.
A question on Physics.SE that starts with "Hey guys" is overwhelmingly likely to be an effort at being informal, rather than an effort at being exclusionary. That particular idiom is neither burn-it-down offensive nor problem-free. Better to remove it, respectfully. But if a debate arises, it's better to have that debate in a dedicated space like this Meta question than in a comment thread. Comments are not good for subtlety, and inclusiveness issues are often surprisingly subtle.