Taylor Swift [1]'s latest 1989 guest will make you want to clap your hands! The songstress welcomed Charli XCX to the stage for a duet of Charli's 2014 hit "Boom Clap" during her Toronto show. The song is great, but damn — how cute are those outfits? Watch the video, then check out Nelly and Haim's recent appearance [2] on Swift's tour!