Clearly they were investors and potential witnesses to Roth's professed largess toward Michael. The details about leaving a small piece for various mobsters was for Roth to show he was thinking two steps ahead of everyone else, and probably paving the way for them to come in after Michael was assassinated. Obviously Fidel had other plans that got in the way.
In "real life" the Teamsters were a big investor in the Havana casinos, no doubt with the help of Meyer Lansky.
Isn't there a line in the Roth birthday scene where he mentions someone putting up some cash with the Teamsters "on a dollar for dollar basis"?
