I don't really call people dude, I use it as more of an exclamation- "Duuuuuuude!" if I'm shocked about something, lol. It doesn't matter if I'm talking to a woman or a man. But I don't say it very often.
I do call groups of people 'guys.' I suppose because.. what else am I supposed to call them? "Guys and girls" is too long, sounds a little awkward.. "people" isn't really as informal..