Let me quote some things from Wikipedia:

The Portsmouth Sinfonia was a real orchestra founded by a group of students at Portsmouth School of Art in Portsmouth, England, in 1970 — however, the Sinfonia had an unusual entrance requirement. Players had to be either non-musicians, or if a musician, play an instrument that was entirely new to them.

Bryars was interested more in experimenting with the nature of music than forming a traditional orchestra. Instead of picking the most competent musicians he could find, he encouraged anyone to join, regardless of talent, ability and experience. The only rules were that everyone had to come for rehearsals and that people should try their best to get it right and not intentionally try to play badly.

and then link to what is now my absolute favorite rendition of In the Hall of the Mountain King. Delicious.