Multicasting provides a scalable solution to deliver a common content to a set of requesting mobile terminals in wireless broadband networks. However, the multicasting rate is limited by the worst channel conditions of the mobile terminals which increases the content download time, and consequently, increases the energy consumed from the mobile terminals' batteries. In this paper, we address the problem of cooperative multicasting where a base station multicasts the content over a cellular network to an optimally selected set of mobile terminals, that in turn multicast the content over multihop connections to other mobile terminals in their vicinity which leads to the formation of cooperative groups. We formulate the joint problem of content distribution, group formation, and interference avoidance as a linear mixed binary optimization problem with the objective of minimizing the total energy consumed by the mobile terminals. Moreover, we propose a simplified multicasting formulation with a close to optimal performance but with a lower computational complexity. Results demonstrate remarkable energy consumption reduction gains in various network scenarios.