It is often necessary for a sink to transmit messages to several nodes that are sparsely distributed in a sensor network. For example, the sink needs to reprogram cluster heads in a tiered sensor network. In this paper, we investigate how to deliver packets to a group of sensors using minimal energy. We propose the grid multicast that builds an energy efficient Steiner tree based on only members' geographic positions. The grid multicast is specially tailored for sensor networks by three design principles: i) it aims to minimize energy consumption in packet multicast; ii) its stateless design does not require intermediate nodes to keep multicast states; iii) it constructs the energy efficient multicast tree without reliance on the global topology. All these characteristics make the grid multicast a suitable protocol for sensor networks with limited resources. Our performance evaluation shows that the grid multicast outperforms other multicast protocols by consuming much less energy.