Toward the end of Geese ’s sold-out Summit Music Hall show Thursday night in Denver, a mosh pit formed a circle during the piano bits in “Long Island City Here I Come,” then waited, waited, waited, finally exploding when the Brooklyn group broke into the song’s raucous climax. The standing-room-only crowd’s spontaneous choreography mirrored Geese’s music: quiet crooning, then something more crazed, then soft again, then loud again.  Geese’s concert at the 900-capacity Summit, across the street