Many chapters can be found in the US, and one was founded for Canadians a few years back by Kaijugal a.k.a. Dawn McKechnie: Fibre Fantasy Artists of Canada (the group is based in Toronto, but opened to all Canadians). Although the ICG itself does not organize conventions or events, members will often participate, and often work at them. Chapters also organize their own events.
The ICG also has a very large collection of photos and videos of costumers, The International Costumers Gallery. It contains over 14 000 items!
"At its core is the Pat & Peggy Kennedy Memorial Library – a storehouse of photographs, video and paper ephemera spanning 70+ years of Science Fiction costuming. Also included are accumulated images from Wearable Arts and performance arts events, Living History and other showcases for creativity."
And of course, there are the mailing lists, discussion groups and forums which are opened to all.
Whether you are looking for other costumers in your area, events you should go to or pictures of costumes past, this is a great resource for anyone interested in serious costuming.