ANMAM appears to be the largest of the professional collaborative organizations I've seen, covering a large number of companies, schools, and individuals operating in France. If you're looking into working or studying in France in the collaborative arts, the ANMAM site is your definitive resource, with news, history of the association, job listings, and programs on the site, to name only a few. If you're not too comfortable reading French, you can always use Google Language Tools or Babel Fish to translate a page into English.
For a sizable collaborative pianists' group that has grown in the wild, be sure to check out People for the Ethical Treatment of Accompanists on Facebook (FB ID req'd).
Previously on the Collaborative Piano Blog: