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.
He is talked into it after the first dinner with Roman and Minnie. While Rosemary is in the kitchen washing dishes with Minnie, Roman and Guy smoke and talk unseen in the living room/parlor. When Rosemary joins them they are close together and go silent, with Guy wearing an expression of secrecy and deep anxiety. And great INTEREST for if you remember he makes a point to say Roman was not boring him but he was very interested in what he had to say and could they get together again to talk about it?
I believe this secret conversation between Roman and Guy is where the idea is introduced to him, with Roman having picked up on his "price" so to speak in that he would do anything, even betray his wife in an unspeakable manner, to become rich and famous. In fact he is shown as such a narcissistic self consumed character that he doesn't even help with the murder of hutch and does not go to his funeral.
I think the deal is actually sealed once Guy is shown that it was real on that the actor who got the role he wanted goes blind. Once he is shown that they are the real and not just two crazy old looks delusionally talking about black magic, and that when he did what they told him he is in all the way.
He's what's referred to as a succubus. I know that most would not agree with me, but of all the characters in this movie, it's Guy that is the most evil. He betrayed his wife, allowing her to be raped by Satan, drugged by satanists, suffer in excruciating pain and also had a hand in murdering someone who was supposed to be his friend, all for fame and money, greed and narcissism. The satanists at least believed they were serving a higher purpose and hurt strangers and people they had no ties to. For me, the character of Guy is far more diabolical and truly evil. Zero integrity.
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).
It is clear throughout the first weeks in the apartment that Guy is a self-centered, uber-macho, 60s man, its his role just as his frail wife Rosemary is a home-bodied simpleton (who just happens to read all the time). Guy is mainly interested in material wealth, he's only interested in Roman once he learns of his connections in the theater. Did you know that he knew Forbes-Robertson!? He often adorns Rosemary in cheesy impersonal gifts, such as cliched roses and clothes. When the couple leave the interview, Rosemary begs Guy to buy it for her, like a child. There are several other scenes involving currency and father-like control: Rosemary bets a quarter that she's pregnant, her "three gold stars", the arguement over the pricing of her doctors, and his constant preoccupation with fame.
Wealth in Guy's eyes means control, since he appears wealthy to Rosemary he has control over her. In truth, and to be short, he betrays Rosemary because he believed that she would adhere to her own gender role and be seduced by material wealth and fame. Rosemary has shown signs of that, she marries a man who is much older than her (financially secure?), she has no job of her own, and she reads magazines about high fashion and furniture (I saw it in a magazine) Its through this that Guy assumes that she would be fine with arrangement made with the cult.