meaning "present with the group" or, more simply, "here".
For example, if you are hanging out with some people and trying to get a friend to join you, you could try to entice the friend by saying:
"Come on, man; everybody's here, plus we got some Hennessey and hot chicks in the mix."
Note: it is not necessary to rhyme "in the mix" with anything, regardless of the above example.
"In the mix" seems to be more often used by hipster white guys who like to use "hip hop" slang as irony; the "I'm so white it's funny when I talk like a rapper" mentality.
Compare and contrast with: up in here.