Joint speech is speech where multiple people say (or sing) the same words at the same time. It has been the focus of my research for many years now. A storehouse of pertinent information is available at http://jointspeech.ucd.ie.
We will discuss Chapters 7, 8 & 9 of the book The Ground From Which We Speak: Joint speech and the collective subject. A pdf of the book is available at this link. You might take a glance at Chapters 1 and 3 as well though.