This paper studies the problem of forming coalitions for dynamic tasks in multirobot systems. As robots—either individually or in groups—encounter new tasks for which individual or group resources do not suffice, robot coalitions that are collectively capable of meeting these requirements need to be formed. We propose an approach where such tasks are reported to a task coordinator that is responsible for coalition formation. The novelty of this approach is that the process of determining these coalitions is modeled as a coalition formation game where groups of robots are evaluated with respect to resources and cost. As such, the resulting coalitions are ensured so that no group of robots has a viable alternative to staying within their assigned coalition. The newly determined coalitions are then conveyed to the robots which then form the coalitions as instructed. As new tasks are encountered, coalitions merge and split so that the resulting coalitions are capable of doing the newly encountered tasks. Extensive simulations demonstrate the effectiveness of the proposed approach in a wide range of tasks.