You can see the choir at the beginning and again at 07:00.
This video shows the choir a lot more. Sam is in the choir on the right called the Voce Chamber Choir. They and the London Youth Chamber Choir on the left sang the first encore, "You Can't Always Get What You Want."
Screen caps of when you can see Sam.
The choirs got t-shirts from Mick Jagger!
Sam is back row on the right!
The back of their shirts say "YOU CAN'T ALWAYS GET TO SING WITH THE STONES... BUT I DID". Congratulations, Sam! :D