I have not heard of a writer's "sorority," but there are quite a number of writer's groups. I'm not sure that's what you had in mind, though.
Sigma Delta Chi: is the Society for Professional Journalists
SABW: Society of American Business Editors and Writers
SATW: Society of American Travel Writers
are just some of them. On this page, you can find many more:
http://www.readersread.com/organizations.htm