1) N-UNCOUNT: also N in pl Membership of an organization is the state of being a member of it.

...his membership of the Communist Party...

He sent me a membership form...

Membership in her church youth group helped develop her political ideas.

2) N-VAR-COLL The membership of an organization is the people who belong to it, or the number of people who belong to it.

The European Builders Confederation has a membership of over 350,000 building companies.

...organizations with huge memberships.

...the recent fall in party membership.

English dictionary. 2008.