Conversation so far:
What are some of Josh Groban's famous performances?
On January 18, 2009, Groban performed as part of the Presidential Inauguration ceremonies
What songs did Josh Groban sing at the 2009 Presidential Inauguration?
performing My Country 'Tis of Thee in duet with Heather Headley
Has Josh Groban ever performed with other famous singers?
On July 1, 2007, Groban performed with Sarah Brightman at the Concert for Diana at Wembley Stadium; it was broadcast to over 500 million homes in 140 countries.
What awards ceremonies if any did Josh Groban perform at?
On February 10, 2008, Groban performed at the 2008 Grammy Awards with Andrea Bocelli in a tribute to Luciano Pavarotti.

Question: Who were his co-stars?
Self-contained rewrite: Who were his co-stars in his 2008 Grammy Awards tribute to Luciano Pavarotti?