The Collaborative International Dictionary of English v.0.48:

Membership \Mem"ber*ship\, n.
   1. The state of being a member.
      [1913 Webster]

   2. The collective body of members, as of a society.
      [1913 Webster]

WordNet (r) 3.0 (2006):

membership
    n 1: the body of members of an organization or group; "they
         polled their membership"; "they found dissension in their
         own ranks"; "he joined the ranks of the unemployed" [syn:
         membership, rank]
    2: the state of being a member

Moby Thesaurus II by Grady Ward, 1.0:

40 Moby Thesaurus words for "membership":
   admissibility, admission, affiliates, affiliation, assimilation,
   associates, association, body of affiliates, companionship,
   company, completeness, comprehension, comprehensiveness, comprisal,
   consociation, constituency, cooperation, coverage, eligibility,
   embodiment, embracement, encompassment, envisagement,
   exhaustiveness, fellowship, fraternity, fraternization, inclusion,
   inclusiveness, incorporation, members, openness, partaking,
   participation, reception, sharing, society, tolerance, toleration,
   whole