Ben Carr has been a full member of the Mighty Mighty Bosstones since 1983 and doesn't sing, play an instrument, or contribute to writing or producing songs. He's strictly a hype man who gets up on stage during live shows, jumps to the beat and flails his arms (calls it dancing), and gets the crowd fired up. He was just a friend of the band and started his hype man spiel back when he was underage and had to either be 21 or a member of the band to get in the clubs they played.
Their fans LOVE him and his only role is to get the crowd fired up and set the tone for the band from the "sidelines." He's considered by the band and fans to be a full and equal band member, but none of the music they put out has any input from him at all.
Ben Carr and Austin Ash are both the hype men of their respective teams and everyone's ok with it.