How did Casaubon, Belbo and Diotallevi meet?

Casaubon and Belbo both frequent Pilade's, a Milanese bar. They met there one night, and Belbo strikes up a conversation with Casaubon, claiming that there are only four types of people: morons, fools, cretins, and lunatics. Belbo invites Casaubon to visit him at his office, where Casaubon meets Diotallevi, Belbo's friend and coworker.