Members of cooperatives
A member of a cooperative is an owner of that co-op. Generally, they become a co-op owner (member) when they economically contribute capital through the purchase of a share of the co-op. By becoming a member, an individual recieves decision making rights in the co-op. The level of decision making rights for member-owners often depends on the size and type of co-op. But in every form of co-op, at minimum, the members elect the co-op's board of directors. In addition, members of co-ops must own that co-op equally - meaning all members may only have one share and one vote in decision making processes.
Membership differences in types of cooperatives
In different types of co-ops, members are made up of different groups of individuals. In a worker co-op, all of the workers are the members (worker-owners); in consumer co-ops (and often "food co-ops" and "community co-ops"), any consumers that elects to purchase a share of the co-op are members (member-owners); in multi-stake holder cooperatives, members are made up of at least two groups of people (such as worker-owners and member-owners); and so on.
Open and voluntary membership
According to the first cooperative principle, membership in a co-op must be "voluntary and open": Co-operatives are voluntary organisations, open to all persons able to use their services and willing to accept the responsibilities of membership, without gender, social, racial, political or religious discrimination. 
In consumer co-ops, this means that anyone who wants to purchase a share may - and thus become a member-owner. However, this is different in a worker co-op, as it would be impossible to allow every person that wanted to be part of a co-op to become a worker. Rather, this means that the co-op cannot discriminate in the hiring process against anyone who wants to join a co-op because of gender, religious, political, social, and other reasons.
(Membership must be "voluntary" because formerly in some countries with dictatorships "cooperatives" had forced membership and were only "cooperative" in name).