Conversation so far:
What are some of Groban's famous performances?
In June 2007, Groban recorded a Christmas album in London with the London Symphony Orchestra and the Magdalen College Choir.
what songs did he sing?
"My Country 'Tis of Thee.
Has he ever performed with other famous singers?
"My Country 'Tis of Thee" in duet with Heather Headley.
What awards ceremonies if any did he perform at?
On May 12 and 13, 2008 At Royal Albert Hall, London, Groban performed as "The Russian.

Question: Who were his co-stars?
Self-contained rewrite: Who were Groban's co-stars?