I think "hey guys" has come to be accepted as a gender neutral term (from my experience in the UK). Somehow "That guy" however is still gender specific...
In your first example I usually use some creative pejorative. In your second example I say "folks".
I try to default to "hey folks" or "hey all" unless it's all male "hey guys" or female "hey girls" or friends for sure "hey (words I don't normally use in public)".
I also try to stick with "person", "someone", singular they, whatever is as flexible as possible until I know what the person prefers to be called.
I do find myself accidentally defaulting to assuming he/she depending on the context. Been trying to correct myself so I don't make people feel unwelcome though. :)
"Some guy couldn't make a right turn at the light for 10 friggin minutes so now I'm late"
Not necessarily a guy driving (let's suppose I couldn't see the driver; I just know their car didn't move for forever) and it's unfair/sexist to men to assume it was a man, could have just as easily been a woman.
"Hey guys, is everyone ready to go to lunch?"
There might well people girls in the group that's going to lunch (there often are) and I don't want to offend them. But "Hey people, " doesn't sound right, "Hey folks," doesn't either, and "Hey everybody," isn't a good substitute either: http://www.youtube.com/watch?v=X8LqozaaB7s&t=1m12s
Anyone have a good gender-neutral word I can use instead?