In this paper, we present a new concept called an identity based ring signcryption scheme (IDRSC). We argue that this is an important cryptographic primitive that must be used to protect privacy and authenticity of a collection of users who are connected through an ad-hoc network, such as Bluetooth. We also present an efficient IDRSC scheme based on bilinear pairing. As a regular signcryption scheme, our scheme combines the functionality of signature and encryption schemes. However, the idea is to have an identity based system. In our scheme, a user can anonymously signcrypts a message on behalf of the group. We show that our scheme outperforms a traditional identity based scheme, that is obtained by a standard sign-then-encrypt mechanism, in terms of the length of the ciphertext.We also provide a formal proof of our scheme with the chosen cipher-text security under the Decisional Bilinear Diffie-Hellman assumption, which is believed to be intractable.