The Shake it Off hitmaker has been keeping fans on their toes with surprise appearances by the stars of her Bad Blood video, Lorde, Nick Jonas, the U.S. women's soccer team and rapper Fetty Wap, among many others, and her latest gigs were no different.

Introducing Fifth Harmony to fans in Santa Clara on Friday (14Aug15), Swift referred to herself as the "sixth member of the group" and praised them for having "one of the biggest smash hit singles in the country right now".

The girls then joined Swift onstage to perform their track Worth It in front of the Levi's Stadium crowd.

The superstar tweeted after the show, "I want to be in Fifth Harmony." She later posted a photo on of their performance, with the caption, "So stoked this pic exists because this was SO MUCH FUN (sic)."

Fifth Harmony's Ally Brooke also shared her joy at the gig in a post on, writing, "I will remember this moment for the rest of my life. Thank you so much, Taylor."

Swift continued the girl band theme during her second show in Santa Clara on Saturday (15Aug15), when she invited British pop stars Little Mix to join her 'squad' and called on Hollywood actress Roberts to make an appearance onstage, alongside veteran singer/songwriter Joan Baez during her hit track Style.