Assembly20070615
Earlier I was a small part of something called "Assembly", 11th in a series of large group improvisations organised by Adem (http://adem.tv/).
The improvisers could bring their own instrument or use one provided by Adem. There were plenty of instruments to go around, and the audience was invited to join the improvisation. There were probably around 20 improvisers. Electric powered instruments (such as laptops) weren't allowed.
Then Adem and others took turns to write instructions on a large blackboard. An instruction would be written, and a signal would be made to enact it. Sometimes an earlier instruction would be pointed at and re-enacted. Some instructions were later modified. Some instructions had loops (play long sounds, then short sounds, repeat). Some instructions were just abstract shapes.
It worked very well. I'm afraid it was rather dark, so my camera phone pictures of the board afterwards didn't come out too well:
http://flickr.com/photos/yaxu/553711876/?rotated=1&cb=1181947867712