We assist law enforcement agencies with investigations of questionable agents, publishers, and others. (accrispin.blogspot.ru)
Writer Beware suggests that writers searching for agents avoid questionable agents, and instead query agents who have actual track records of sales to commercial publishing houses. (nielsenhayden.com)
This isn't really a membership, but I'm including it anyway, because a surprising number of questionable agents claim it (some even insist on being called «Doctor»). (accrispin.blogspot.ru)