Secrets of the Witch

From the introduction:

This book focuses primarily on covens: groups of casters who work together and pool their magic in order to achieve greater feats of arcane mastery than they could manage on their own. The cornerstone of the coven mechanic is a feat, Coven Initiate, which allows characters to create and join covens. The book also contains a number of feats and spells designed specifically to benefit casters who choose to form covens, as well as a base class for characters who wish to play as green hangs, a monstrous race known for their ability to form powerful covens.