Who joined Radiohead for “15 Step” when they first played the Grammys, in 2009?

The USC Trojan Marching Band, 36 of them, the university band that played on Fleetwood Mac's “Tusk” in 1979. In the rehearsal film, Thom Yorke tells the drummers their snare drums are too busy. In Rainbows won two Grammys that night, 8 February 2009.