The show was just on and I caught this scene because I remember your question. It's hard to say because there's not enough information, but I'll say what I think.
Is there any evidence that Doctor Hill was a member of the cult that
Rosemary was fleeing from?
I think it's unlikely that he's closely associated. When she calls him on the pay phone, talks to his answering service and finally gets him to call her back, he doesn't want to see her. He's home, not in the office. He even mentions taking a nap before he sees her. If he was an in the know cult member, I don't think he'd have been so put off by her request to see him urgently. To the cult members, she's so important they try not to let her out of their sight.
It's tough to read Charles Grodin in the scene, but he plays the role with a dry and disinterested manner. He listens to her, but doesn't show much interest. He only says enough to keep her talking and mostly listens.
He lacks the "I'm watching you" vibe that the other characters have.
His two moments of interest during the conversation were when she shows him the book of spells and explains to him that they cast a spell on someone. He looks at the book with some interest, checks the cover and asks, "Can I keep this"
And when she mentions Dr. Sapirstein, he really perks up. She asks if he knows him and he says they met once or twice (which wouldn't explain the perking up when he hears that name.)
It's more likely he didn't know who she was when she called but he may have known something about Dr. Sapirstein and spells, but not of the plans of the cult. That's my guess.
It's also entirely possible that he simply called Dr. Sapirstein as a colleague given that he was bothered by an old patient to show up to the office when he was already home and he was the one who suggested a lunatic asylum that Dr. Sapirstein threatened her with when he came to pick her up.
I don't think it's possible to deduce with certainty, but his interest in the spell book suggests he knew something about spells and witches, but he wasn't in the know about the baby she was carrying.
why didn't he just take Rosemary to the hospital?
She wasn't in labor, so there was no physical need for her to go to the hospital. Her due date was in a few days, but, in theory, what would he say?
Dr: "I'd like to admit my patient to the hospital"
Hospital: "Is she in labor"
Dr. "No, but she's very upset"
It wasn't really a hospital situation. That said, at the end of the conversation, Dr. Hill says something like "I can try to get you into Mt. Sinai tonight. It'll be difficult". Whether that was true or just something he said to get her to go to stay in his office and go to sleep is unclear.
From the Dr.'s point of view it was a very odd story that he doesn't usually deal with. That's why a few explanations work.