Authorship

For the artworks which invite viewers to participate, specially for those need participants to create contents, there is a question arising, who is the author of the artwork? The artists or the participants? I like Joseph Beuys's idea which he suggests that "everyone is an artist". When the viewers are participating the artwork, their contributions became a part of the artwork. Indeed, they are the author of the artwork. For most of the contemporary art, artists are more focused on the conceptual idea of the work, viewers' contribution to the work is still under the artist's conceptual idea. So, artist is the author, and viewers become the author as well.