Writers in any genre literature tend to form AGs. This is true in two broad senses: conventions and language are shared; and lives are shared.
An AG, in the second sense, is a communication matrix linking people of like mind who wish to do or to appreciate similar things. The group of writers and readers who focused on and corresponded with H P Lovecraft is clearly one, though most of them did not meet physically. Meeting regularly to drink and read aloud to one another, C S Lewis, J R R Tolkien and Charles Williams informally declared themselves members of an AG they called the Inklings. The Society for Creative Anachronism informally assembles writers and readers to act out medieval fantasy routines. The James Gang is a post-facto description of many writers who share an interest in the Ghost Stories of M R James, and who often consciously aspire to write like him. In Minneapolis, USA, a group of writers formed itself into a social and creative scrum they called the Scribblies; in Toronto, Canada, a similar group called itself the Bunch of Seven (> Canada). Both groups have generated Shared-World tales or sequences (though it stretches the term beyond usefulness to describe a shared-world series as an AG enterprise). In Oxford, UK, the Pieria group included Robert Holdstock, Garry Kilworth, David Langford, Michael Scott Rohan and Allan Scott.
Writing workshops could also be described as AGs: certainly the writers who have been involved in the Clarion Science Fiction Writers' Workshop and the US or UK version of the Milford Science Fiction Writers' Conference are members of formidable AGs. [JC]