No, "si" is only used as a positive response to a negative question. A: Tu ne vas pas au club, n'est-ce pas? //You're not going to the club, right? B: Si! //

No, "si" is only used as a positive response to a negative question. A: Tu ne vas pas au club, n'est-ce pas? //You're not going to the club, right? B: Si! //Yes! ; this means B is going to the club B: Oui! //Yes! ; this means B is agreeing with A, indeed B is not going to the club; in practice this is probably somewhat ambiguous B: Non! //No! ; probably this also means that B is not going to the club, though it could also mean that B is contradicting A: B is going to the club. In contrast, if A had asked "Tu vas au club?" (Are you going to the club?), B wouldn't normally answer "Si", since "Si" only makes sense as a response for a negative question.