Like Johnny Ola said, "Hyman Roth always made money for his friends."
As for the other investors in the Cuban casino deal, I really don't think they can or would have blamed Roth for that. Although an argument could be made that Roth let his greed get the better of him by turning a blind eye to the Castro threat.
In real-life, Meyer Lansky thought that he could continue doing business with Castro, even secretely financing his rebellion. It sounds plausible to me that Roth was playing the same double game.
We're never told in the movie that the other guys with Roth at his birthday party were investors, although it's not unreasonable to expect that.
The movie strongly implies that the "Lakeville Road Group" were one of his other investors.
