This paper presents a formal analysis of social interactions within multi-agent systems. The fundamental building blocks of such systems are social agents which can be individuals or aggregations of agents whose structure can be formally characterised in terms of roles and relationships between them. Agents are free to join social agents while in pursuit of their own objectives, but at the same time they have to balance their preferences and their commitments. Stability and regulation of behaviour within a multi-agent system and within social agents is accounted for by means of commitments, obligations and rights.