As I see it, nobody was there except Batman. But saying "stand back" to Batman would be redundant since he wasn't approaching the Joker at all. Besides, "Bat Jokes" would've been more appropriate in his situation.
And he said it at the exact moment the camera FIRST laid focus on him (i.e. OUR eyes first laid focus on him).
So, in a sense, he was telling us to stand back.
Or was I just imagining things?
He did have a history of breaking 4th walls, correct?
It sounded to me when he said "I'm watching you all. Even you. Yes, you!" over the loud speaker he was just talking to any one in the Asylum who was listening to make them paranoid, I did not get the impression he was talking to the gamer