Jack, obviously Chris talks about that Yahoo group "abcusers", which is also mentioned on the official site http://abcnotation.com/. It can be read without registration. I just browsed through some recent threads containing discussions as mentioned by Chris, very technical. Members include (persons who sign as) Chris Walshaw and Jean-François Moine, though posting infrequently. I wonder whether "abcusers" is really the "Inner Circle" - it looks very public, in spite of requiring registration for posting.