Joni Mitchell‘s Los Angeles Joni Jam shows on Saturday and Sunday, her first Los Angeles headlining dates in over 24 years, had her joined by Brandi Carlile, Annie Lennox, Jon Batiste, Blake Mills, Robin Pecknold, Marcus Mumford, Celisse, Taylor Goldsmith, Lucius, and Allison Russell, among others — but that was only on stage. Turns out there were even more guests backstage, some of whom joined Joni for a rendition of Elton’s “I’m Still Standing” (which she also performed onstage on both nights). Elton, Russell, Mills, and Tim Hanseroth can all be seen in the video clip, which Carlile posted on her Instagram, as can Chappell Roan and Lucy Dacus:…