Banesfinger
Explorer
Kimmuriel Oblodra.
The only person in the chamber at the end that knows Gromph summoned Demogorgon, explicitly stated in the book, is Jarlaxle. There was a telepathic conversation between Cattie-brie and Gromph but we don't know what they had talked about. Cattie-brie only mentions she knows Gromph needs help. Maybe he said he was cast out? We don't know at this point. I'm sure the vagueness was intentional as there were different authors working on different books (OotA and Archmage)
Obviously Kimmuriel (psionicaly) knows what happened (hardcover, page 351-352), he says "Gromph...summoned... [whisper] Demogorgon."
And likely a 4th person also knows that Gromph summoned Demogorgon: Matron Mother Zeerith Xorlarrin (Hardcover: epilogue, page 361-362), the candid conversation between Kimmuriel and Gromph "What did you do?" was done in front of the Matron Mother.
And Gromph absolutely knows what he did (page 361): "Gromph cleared his throat. He didn't much like the vulnerability his error had brought upon him, but he had, after all, summoned Demogorgon to the Prime Material Plane."
Your are correct, it would be speculation to guess what Gromph (telepathically?) said to Catti-brie, but for her to say "You need us" (page 355) to one of the most powerful wizards of the mortal realms, it would have to be a very big/powerful request indeed. And if Catti-brie knows, you can guess the rest of the company would be told as well.
The point is, we won't find out until April 2016 (when the next book is out). Obviously, most people will be done (or well into) the adventure by then.
While I can obviously 'fudge' this point, I'd like to keep intact with the multi-genre story line if at all possible. (Many of my players also read the books, play the MMO, and have plans to purchase the Sword Coast Legends computer game (all featuring coinciding story-lines).