From the Wiki description:
The Visitors constitutes the performance of a song written by Ásdís Sif Gunnarsdóttir, Kjartansson’s ex-wife. The piece is displayed across nine different screens, each featuring musicians or artists either by themselves or in groups in different rooms of the house at Rokeby Farm, or on the grounds outside, performing simultaneously but separately. One screen features Kjartansson by himself. Others featured in the piece include friends of Kjartansson, both from the artist’s native Reykjavík and elsewhere, as well as residents of Rokeby Farm, where the piece was filmed.
Eventually, the musicians move from their isolated spaces — libraries, bedrooms, a bathtub — and congregate in a downstairs room before filing out onto the grounds, across a beautiful meadow, dancing and drinking and continuing to sing.
It’s about an hour long, and is utterly, amazingly beautiful.
The WaPo has a great interactive oral history of the piece that is 100% absolutely worth your time. (h/t to Erin!)