New collaborative applications relying on a MANET need multicast to perform efficient group communication. Application Layer Multicast is seen as a good alternative to the traditional network layer routing protocols because of its flexibility and ease of deployment. However, existing application layer multicast protocols for MANETs are not designed for collaborative scenarios. In this paper, we present OMCAST, a new overlay multicast protocol for mobile ad hoc networks that is specifically targeted for collaborative scenarios. OMCAST benefits from the broadcast nature of the wireless medium in order to minimize network delay and congestion. Evaluation shows that OMCAST achieves better network performance than current message delivery approaches.