Hey chat!
This new use of chat is to be the equivalent of guys or gang, as in Hey chat, where are we going tonight? It derives from its use on social media where it serves as a form of address for the people in the chat group viewed collectively.
But, as is often the way with these new words and new meanings that pop up in the online world, this one has escaped to the real world according to John McWhorter who writes on language for the New York Times. He spent a lot of the summer holiday listening to his daughter and her tween friends — and asking for explanations. Meanwhile, back in Australia, we seem to be still arguing about whether Hey guys is acceptable or not, exclusionary or not, with suggestions that Hey team or Hey folks or Hey peeps would be better. I cannot imagine myself using any of those expressions so perhaps Hey chat will save the day.