The two veteran actors confessed their love for Swift last month and begged her to invite them into her inner circle, which includes pals like models Lily Aldridge, Karlie Kloss and Cara Delevingne, Selena Gomez and Girls star Lena Dunham.
During a recent US radio interview the thespians recited the lyrics to Swift's most recent hits.
Stewart took to Twitter afterwards, writing: "So @IanMcKellen recited Bad Blood and I did Blank Space on @NPRAskMeAnother. May we join the squad, @taylorswift13?"
The singer was alerted to the correspondence and responded, "Thanks for reciting my lyrics, @IanMcKellen and @SirPatStew! You've made my day. You two are ULTIMATE Squad Goals."
Swift also posted several photos of the actors, including one of Stewart's wedding day in 2013, which was officiated by McKellen.
Thrilled Stewart couldn't contain his excitement in a follow-up tweet, writing, "OMG I DIE (no really, I might. I'm old.)"
Swift's tour dates have become all about her surprise guests - she has recently shared the stage with the stars of her Bad Blood video, the US women's soccer team, rapper Fetty Wap and Seattle Seahawks quarterback Russell Wilson and his girlfriend Ciara.
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- WENN