A number of peer to peer (P2P) systems as well as mobile ad-hoc networks (MANETs) fall into the category of ad hoc groups. Their needs for security have been widely recognized, so some signature schemes had been introduced for the ad hoc group. On the basis of analyzing those schemes, we propose a threshold ring signature from bilinear pairings for ad hoc groups. On the basis of analyzing the proposed scheme, we conclude that our scheme meets with the three requirements of threshold ring signature: correctness, anonymity and security against adaptively chosen message attacks in the random oracle model assuming the computational Diffie-Hellman problem (CDH problem) is hard to solve. Moreover, the small size on the signature, robustness, multi-signature and distributed computing characteristics of the proposed scheme, are useful for ad hoc notes to save communication overhead, memory size, computation, energy consumption and safety from widely known attacks