Both Sedway and Greenbaum were associates of Lansky. Sedway operated out of NY, Greenbaum was with the Outfit. Tony Accardo specifically asked Greenbaum to manage the Riviera Hotel in Vegas. According to many stories, Greenbaum was murdered in '58 on orders from Accardo because he was skimming.

We'd have to infer that both were involved with Siegel's hit--especially after they showed up at the Flamingo about an hour after he was killed--but the triggerman or men have never been identified. The movie would have us believe that Mickey Cohen pulled the trigger, but I doubt it.