I want to say Claudius AND Gertrude because she's in on it as much as he is although their motives are slightly different; she wants Hamlet to cheer up while he wants to find out if Hamlet's onto the murder.
Anyway, R&G are guys Hamlet presumably hasn't seen in a while, maybe years. Meanwhile, there's Horatio right in the court with Hamlet's full confidence and affection. Do Claudius and Gertrude bypass Horatio because they sense he ain't going to do no undercover work?