The "BOC" as it's known. Approximately 850 members with affiliates around the world. Worth the joining cost. Peter McGough is the North American registrar, he is based in Canada.
Bristols of America/Arnolt Registy
The offline version of the Club is spearheaded by Shawn Thomas with occasional assistance from myself. There is no fee for this club. A somewhat irregular circular is sent out by Shawn.
Hugh Knox and Bill Russell are some of the officials of this club.
The New Zealand branch of the BOC.
The BOC also has various registrars for models and regions. Use the BOC link above to get further details.