"Hey man" is probably the most natural and casual sounding, but only if you're talking to a guy, and you'll sound like a little bit of a tomboy, which can be good, and kinda fun, if you don't mind that. Otherwise you can just say "Hey" or "What's up.", which is how most girls have said hi to me in my life I think. "Hey buddyyy" would sound really friendly I guess, like being surprised to see an old friend or something (either gender).. or it can also also sound angry if you say it angrily (more of a New York East Coast thing I think?), as in like, "Hey buddy, what the heck do you think you're doing?!" "Hay girl!" or "Hay girl hayy!" can be a really friendly and girly way to say hi to a close female friend, similar to "Hey buddy", but you also might sound like a bit of an airhead to some people (like a "Valley girl" kinda, that sort of feel). I say "Hay girl hay" to girls because it makes me sound like a girl or a gay guy and I find that funny for whatever reason. The word "dude" has a strong "bro" feel to it, so you'll sound even more like a tomboy/dudette/brosette if you say that than if you just say "Hey man". I remember foreign women who felt comfortable talking to me (family friends or whatever) calling me "dude" when I was a kid and it was very friendly and disarming and kinda funny (intentionally). "Dude" is also a good way to start a sentence when you suddenly think of something, as in like, "Dude! Steve's parents are out of town this week. We should totally throw a kegger!!!" or "Dude! What if birds aren't really singing.. What if they're just screaming cause they're afraid of heights?!" Obviously you'll totally sound like a brosette if you talk like that, which is great if you want to make friends with bros, brosettes, and people who talk like them. I'm not really a bro per se but I pretty much talk like one most of the time.

It's more natural to say "hey guys"

Personally I wouldn't use either whether I were male or female. Both are very American, and specifically American I think. In Australia we use 'Mate!' (but I don't), and in England a Cockney version is Old Plate (rhymes with Mate). Use jiahuo instead! Guy! (perhaps)

Hmm~ some of my friends say: Hey Chika! Hey Girl! We say these too: Hey Buddy! Hey Dude! Or if you have your own nickname for them like: Hey Goobers! Haha~

Thank you!!!