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. But:

Roth's speech at his birthday party was designed to create an alibi for him--while at the same time keeping the other guys dependent on him. So, if Roth's plan had worked, and Michael had been assassinated after the Presidential New Year's Eve party, Roth could have told the authorities (and any Corleone members looking for revenge): "What, me arrange for Michael's death? Why, I loved him like a son. I was gonna turn everything over to him. Just ask any of those thugs who were at my birthday party." And, by designating some of his holdings to others, like the Lakeville Road Boys and Eddie Levine of Newport, Roth was making sure that no one would get ideas about whacking him after he took care of Michael. He would have shown them that he was still in charge--and with all-powerful/all-greedy Michael out of the way, they'd be sure to get a piece of his action.