Once, Japanese improvisation conductor Otomo Yoshihide came to my city and there was a call-out for musicians to come along to a rehearsal and concert. Not having an acoustic bass, I went along with my acoustic guitar because there were no facilities for electric instruments. About 60 people turned up.
"Rehearsal" involved learning some simple hand movements [fast, slow, quiet, loud] and getting into the mood of improvisation. There were all sorts of instruments - violins, guitar, drums, ukeleles, a few banjos, some singers, horns, xylophones, etc.
After "rehearsal", we went to a nearby performance area and we played for half an hour doing pure improvisation. It was a very interesting experience!