Most of us can probably name a grandfather or great-aunt who was active in a chapter of a national association. My own uncle was a member of the Benevolent and Protective Order of Elks. Yet how many of us can say the same about ourselves? Marshall Ganz. Photo: Harvard University/Justin Ide. As voluntary associations fade from our cultural landscape, political participation is threatened, especially on the left, says sociologist Marshall Ganz. And, he says, that trend is undermining the environmental movement, which has long depended on engaged members to carry its banner. That's why Sierra Club leaders recently turned to …