Don’t Think Twice, Mike Birbiglia’s second feature (following Sleepwalk With Me), starts with the rules of improv comedy. One of the rules: It’s all about the group. If you break that rule, everything falls apart. Even — or especially — if you break it by attaining the success every member wants.

Birbiglia’s fictional troupe is the Commune, a six-person team with a solid run at a fictional New York venue. Miles (Birbiglia), who taught some of his colleagues, is still holding out for his shot at Weekend Live, the movie’s version of SNL. Bill (Chris Gethard), the quiet one, is still in the shadow of his very different father. Sam (Gillian Jacobs) and Jack (Keegan-Michael Key) are a couple; they riff off each other everywhere, comfortable and cozy. Allison (Kate Micucci) and Lindsay (Tami Sagher) are less developed, but Allison draws charming cartoons and Lindsay smokes a lot of pot and lives with her parents. It’s believable.

And it all starts to wobble when Jack and Sam score auditions for Weekend Live. Birbiglia traces the bitterness and the hopes that pass from teammate to teammate, whether it’s Miles’ resentment that the student has eclipsed the teacher or Bill and Allison’s quiet focus on their writing. (A tragedy plot runs in the background, giving the movie a chance to show just how much these people love each other, even if they can’t always be happy for each other.)

Miles, who is still sleeping with students and living in the Commune’s, uh, communal house, gets a lot of screen time and a little more plot than he needs; the movie’s single off note is Liz, the old friend with whom Miles reconnects. Liz is barely a character, just a combination of Eat Pray Love and that Sex and the City episode where Carrie scoffs at a twentysomething’s apartment — and her presence in the story is frustratingly all about Miles.

But balancing that is Gillian Jacobs’ thoughtful, sweet, honest performance as Sam. It’s hard to talk about how good Jacobs is in this role without spoiling the movie’s greatest pleasures, but there is a moment when Sam winds up onstage alone, and what she does with that bare stage — the way she takes it and makes it hers, just hers, no need for rescue or help or saving — is transcendent.

The Commune is indeed a team, but Birbiglia’s movie gets at something deeper than just the magic of a performer’s stage family: the way each person on that team has his or her own rhythm. Sometimes they sync up, and sometimes they all fall out of step with each other (though never onstage). Through the arguments and the sadness and the jealousy, Don’t Think Twice remains a quietly jubilant film, celebrating the way improv makes something — a laugh, a family, a partnership, a career — out of nothing.

We might hate it when our friends become successful, but that doesn’t mean we hate the friends themselves. (Opens Friday, Aug. 26, at Bijou Art Cinemas)