I don't know if Langtry is on the Grey Council. In fact, I hope he isn't. I like the idea of several wizards on the Council having to run everything behind Langtry's back. However, he could be Grey Council.
I do not think we will see any members of the Grey Council through most of Twelve Months. The exception might be a scene between Ebenezer and Harry, as Eb will lose it when he hears Harry and Lara are making public appearances together.
Here is where I think we could be introduced to a new; for us, member of the Grey Council. We know that LTW agreed to talk to unspecified others, to speak for Harry, to ask permission to tell Harry more about being Starborn. LTW told Harry he would have an answer in a year. That puts this meeting near the end of Twelve Months. I think it is a good guess the others LTW will speak to are the Grey Council.
Much was made of LTW and Matha Liberty's advancing age in Peace Talks and Battle Ground. River Shoulders specifically said that LTW was nearing the end of his journey. So, either LTW shows up with another member of Grey Council and introduces him or her to Harry; because LTW knows he won't be around much longer, and together they tell Harry what it means to be Starborn or LTW passes away, leaving Harry thinking he will never learn about being Starborn only to have this unknown member of the Grey Council show up and introduce him or herself. I suppose if Arthur Langtry is a member of the Grey Council, this would be a near perfect way to show us. It would shock the hell out of Harry; plus, my guess is the knowledge Harry will learn should be jaw dropping as well. It has to be to be such a secret.