When did Guy begin to take part in the plan to use Rosemary to give birth to the devil's son? It's weird because at the beginning of the movie he didn't want to buy the department.
The exact reason reason why he joined the cult isn't shown in the movie.
Guy forms a bond with the Castevets after he and Rosemary go to their apartment for dinner.
Shortly after Guy lands a role in a play when the actor who was originally cast suddenly and inexplicably goes blind.
It can be implied that the cult made the actor go blind through rituals. Another example of this happens when Hutch goes into a coma later on in the film after Rosemary goes to him for help.
Therefore I think the most likely reason is that he was seduced by promises of fame and success from the Castevets and this is why he joined them.
We don't know precisely when he joined but it was some time after the first dinner with the Castevets and when he got that first role. So it was fairly early in the film and before the sex scene with the devil.
There are several scenes before Rosemary's impregnation that allude to Guy's reasons to join the cult. When him and Rosemary are being showed the apartment, the landlord asks Guy if he is a doctor. He lies, saying yes, when in reality he is a struggling actor. He was Luther and played a small role in Nobody Loves an Albatross This means that he's a liar, and a mediocre one at that. His role as an Albatross also echos his impending betrayal (albatross' represent burdens and obstacles).