"Get Together" (sometimes called "Let's Get Together") is an old song from the '60s. A lot of people covered it back then, but I think fairly famously by Jefferson Airplane. The Youngbloods also had a big hit with it.
Beck performed at Neil Young's Bridge School Benefit concerts in 2011. The show ended with all the musicians on stage. Neil Young sang the song and played guitar while Beck, Eddie Vedder, Dave Matthews, Arcade Fire, and their bands all sang along during the chorus.