"guys" is slang since about 1970 for 'everybody', probably from the American Mid-West. My first experience of the term as distinct from meaning 'males' was the

"guys" is slang since about 1970 for 'everybody', probably from the American Mid-West. My first experience of the term as distinct from meaning 'males' was the 'Mary Tyler Moore Show' when Ted Baxter, one of the newsreaders, always entered the room saying "Hi, Guys" when saying 'hello' to all the people in the room. I remember thinking back then that it 'didn't sound right'. However over time, I found myself using the term more and more.