This paper addresses the problem of efficient trajectory planning of a group of autonomous systems. The trajectory plan is targeted to satisfy the dynamic equations, path and actuator constraints, terminal constraints, and minimize a given cost criteria. The proposed approach builds on two features of dynamics of a group: (i) the governing equations for each member of the group are differentially flat, and (ii) the inequality constraints have special structures arising out of proximity constraints between members of the group. Under these assumptions, the trajectories of members of the group are coupled with respect to their neighbors so that the trajectory planning of the group reduces to trajectory planning of the leader. An illustrative example of unmanned vehicles in a formation is presented.