Jeremy Shamos, Sam Robards and Mark Blum have joined the cast of “The Assembled Parties,” the new play by Richard Greenberg bowing on Broadway in the spring.Shamos, currently appearing in the revival of “Glengarry Glen Ross,” plays a guest at a 1980 dinner party on the Upper West Side. With Jessica Hecht and Judith Light playing, respectively, a former movie star and her sister-in-law, Robards is on board as the husband of Hecht’s character while Blum appears as the husband of Light’s character. Three more parts remain to be cast in the Manhattan Theater Club production, to be helmed by a.d. Lynne Meadow. Greenberg has a busy season ahead of him, with “Assembled Parties” on the sked alongside a Broadway staging of his adaptation of “Breakfast at Tiffany’s,” aiming for a February start, and the spring Off Broadway berth of “Far From Heaven,” the tuner version of the 2002 pic. MTC staging begins preview March 19 ahead of an April 17 opening.