The main advantage of ring signatures is to ensure anonymity in ad hoc groups. However, since a group manager is not present in ad hoc groups, there is no existing way to identify the signer who is responsible for or benefit from a disputed ring signature. In this paper, we address this issue by formalizing the notion of ad hoc group signature. This new notion bridges the gap between the ring signature and group signature schemes. It enjoys the same advantage of ring signatures to provide anonymity whilst not requiring any group manager. Furthermore, it allows a member in an ad hoc group to provably claim that it has (not) issued the anonymous signature on behalf of the group. We propose the first construction of ad hoc group signatures that is provably secure in the random oracle model under the Strong RSA assumption. Our proposal is very simple and additionally, it produces a constant size signature length and requires constant modular exponentiations. This is to ensure that our scheme is very practical for ad hoc applications where a centralized group manager is not present.