I agree with your answer, but can't this be an exclamatory question (since it invites the hearer's agreement to something on which the speaker has
strong feelings)? ( (Quirk, page 825)
Hi, Ahmed—I don't see this as an exclamatory question (like "My, hasn't she grown!" or "Boy, am I hungry!"). The question asks whether the interloctor would also like some juice. If I said "I want to get some juice. Want some?" and received "Yes" in response, the next step would be either to get both myself and my interlocutor some juice, or to take my interlocutor with me to get some juice. If the answer were "No," I would go for juice by myself.