Stewart, 23, and the crew were spotted filming recently by their Twitter followers.
"Kristen has always wanted to direct, and no better person to do that for than Sage," a source told E! News.
Sage is a friend of Stewart and her ex-boyfriend Robert Pattinson.
"We are shooting our music video and need you to come party with us," Sage informed her fans via Facebook Tuesday.
"Bring your friends, lovers, enemies, and mothers!," she wrote.
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