While interest groups use a variety of techniques to exert influence, coalition strategies are the dominant lobbying technique. However, many questions remain about such coalitions. This paper is the second in a series of social network analyses of purposive and coordinated interest group relationships. We utilize a network measure based on cosigner status to United States Supreme Court amicus curiae, or friend of the court briefs. The illuminated structures lend insight into the central players and overall formation of the network over the first several years of the 21st century. The factions are tied together by various central players, who act as hubs, leaving a disparate collection of organizations that work alone. Using an exponential-family random graph model (ERGM), we find that graph-theorectic and organizational characteristics, such as size and budget, as well as policy interests explain interest group network formation.