Members of a co-operative are individuals (natural persons) or corporate organisations (legal persons) who are users of a co-operative's services or participate in its business enterprise as consumers, workers, producers or independent business owners. The type of members will depend on the nature of each co-operative. Members are also the co-operative's stakeholders, co-owners and co-decision makers with authority over major business decisions.

The centrality of membership can be seen in all seven Co-operative Principles and it is one of the most powerful concepts. In essence, membership means that there should be a special relationship between the co-operative and the people whose needs it is established to serve. That relationship should define the business conducted by the co-operative, affect the way it does business, and shape its plans for the future. A recognition of the centrality of membership means that co-operatives will be committed to particularly high levels of service to members, who are the primary reason for their existence.

The identity of members democratically in control of a co-operative is an organisational trait that differentiates co-operatives from shareholder-owned and/or joint-stock investor-owned companies where customers, investors, workers and managers are separate and distinct. Co-operatives are user-driven organisations (or worker-driven in worker co-operatives) established by, owned by and operating to meet their members' common economic, social and cultural needs and aspirations.

Many commercial enterprises imitate membership by inviting customers to join wholesale clubs, commercial loyalty schemes, frequent flyer programmes, or club card schemes. This demonstrates the business value of membership in competitive markets. Membership of a co-operative is fundamentally different to these arrangements, which are no more than marketing tools that do not grant 'members' rights of ownership or any involvement in business decision-making.