It’s Will Ferrell’s party, and we’re all invited. On Oct. 7, the actor made a surprise appearance as guest DJ at a Sigma Alpha Mu frat party at the University of Southern California, his alma mater. Getting the party started ahead of the football team’s game against the University of Arizona, Ferrell hopped behind the turntables and blasted a medley of hits, including “Eye of the Tiger” by Survivor and “Mr. Brightside” by The Killers. Ferrell also played “N— in Paris” by Jay-Z and Kanye…