The beauty behind Toronto's indie collective, Broken Social Scene, is that it's always been about playing music with friends. Because all of its members are involved in other projects, the band consistently has maintained a kind of artistic open-door policy. It allows its members to come and go as they please, thereby shrinking, swelling and rotating the group's numbers into a new formation every time it records an album and goes on tour. With members Kevin Drew and Brendan Canning serving as the closest things to constants, the band delivers plenty of sing-a-long rock, untidiness and camaraderie. Recently contributing tunes to the awesome Edgar Wright film, “Scott Pilgrim vs. the World,” as well as releasing its latest album, “Forgiveness Rock Record,” Broken Social Scene is scheduled to play the National on Saturday, Sept. 11, at 8 p.m. $20-$25.