The performers are Hugh Jackman, Russell Crowe, Anne Hathaway, Amanda Seyfried, Sacha Baron Cohen, Helena Bonham Carter, Eddie Redmayne, Aaron Tveit and Samantha Barks. Fittingly, given the roles they played in the film as comedy foil con-men, keep an eye on Cohen and Bonham Carter later in the video, having a fun time among themselves, as he also records the whole thing on his phone,
Also there are music supervisor Stephen Brooker, composer Claude-Michel Schönberg next to him, director Tom Hooper standing in the back with his arms crossed, producer Cameron Mackintosh (who produced the stage production), and next to him original French lyricist Alain Boublil with a scarf around his neck.
And here's the end result on the Oscar telecast. You can check for yourself that it does appear to be sung live. The sound quality is poor, the user seemingly having recorded it second-hand, probably with their cel phone, not from the direct TV feed. But it comes across well enough.
By the way, as long as I was giving credit above, the English lyrics to the show were written by Herbert Kretzmer.