In this paper, we investigate the multiagent planning problem in the presence of cooperative actions and agents, which have their own goals and are willing to cooperate. To this end, we extend the action language A in [12] to represent and reason about plans with cooperative actions of an individual agent operating in a multiagent environment. We then use the proposed language to formalize the multiagent planning problem and the notion of a joint plan for multiagents in this setting. We discuss a method for computing joint plans using answer set programming and provide arguments for the soundness and completeness of the implementation.