The enlivening pulse created by Radcliffe, Mendez, and Groff gains strength and drive through the production’s rock-solid ensemble. Gilmour (also the costume designer) dresses them in softly period, unified swaths of color as the play moves back in time — blues, then beiges, then, in the lavish, La Dolce Vita–ish early ‘60s, in hard black and white. There’s something smart happening here: Groff, as Frank, wears varyingly sophisticated versions of the same white shirt and black trousers throughout the show, but at the top of Act Two, as Gussie (not yet his wife) seductively introduces him to “The Blob” — a pulsating swarm of influentials, “the ones who know everyone that everyone knows” — Frank’s clothes match the company’s for the first time. He is, whether consciously or not, getting sucked into something. No — it already has him.