There wouldn't have been a threshold from the never never, would there?
Also Harry is her bitch, so she probably had a standing invite until he settled his debt. (well, he wound up being even more in her debt by becoming her knight, but, y'know)
Slight correction, that was Summer Fire, not Hellfire Harry threw, which was given to him by Lily. So either Mab was working with her on that, or Mab didn't control every aspect of everything.
Oops. I stand corrected. Maybe. But isn't his fire always laced with hellfire while Lash is on board? Another thing to check with my next rereading. If he doesn't get Peace Talks out soon I'm going to have My Dresden library memorized.