"How to Grow a Band," About the Punch Brothers Directed by Mark Meatto 1 hour 28 minutes; not rated Nobody smashes a guitar, pitches a screaming fit or, for that matter, overdoses in a swank hotel room in "How to Grow a Band." No, in this notably well-mannered music documentary from Mark Meatto, the reason a departing bassist peels off is to devote more time to his family. The band is the Punch Brothers, the bluegrass brainchild of the 31-year-old mandolin maestro Chris Thile, and the film's modest chronicle of creation tracks this young fusion group, onstage and behind the scenes, as it hones a sweet-and-lonesome long-form sound. Shown performing in 2008 and 2009 in Scotland and England and then in the United States, the Punch Brothers play their exacting leader's flagship composition, a shifting four-movement suite (with limited vocals) titled "The Blind Leaving the Blind." Once a star of the platinum-selling bluegrass trio Nickel Creek, the elfin Mr. Thile heads up a skilled quintet whose members defer to the virtuoso but not without job anxiety and measured disgruntlement (aired out of his earshot). Mr. Thile's painstakingly articulated aspirations ("The second and third movements introduce the concept of dating into my life") suggest that the music is best heard unglossed. The band's string arrangements have a pleasing shuffle and canter but sound far afield from a bluegrass hoedown. Mr. Meatto's framing tends to catch the musicians" give-and-take, merging and dueling notes onstage, especially at a Manhattan concert that is the film's highlight. Ego struggles and innovator's laments (nobody gets us!) are a refrain in many band documentaries. "How to Grow a Band" adds a modest but effective entry to the genre's back catalog.