One of the main challenges in multi-agent systems is the coordination of autonomous agents. In order to achieve this coordination, the agents are considered to be part of what we call a group (e.g., organization, institution, team, normative society, etc.). Our goal is to enable an agent to reason about the implications of being part of a group: what does it gain or lose, what are the constraints imposed on its behaviour. The theory of social power has been proposed as a paradigm to describe the agent's behaviour. In this paper we use this theory, we formalize it and we extend it to include group-related aspects. We then show how, using this theory, an agent is able to reason about the constraints imposed on its behaviour by the group, for example to decide whether it should enter or not a group.