The Collaborative International Dictionary of English v.0.48:

Companionship \Com*pan"ion*ship\, n.
   Fellowship; association; the act or fact of keeping company
   with any one. --Shak.
   [1913 Webster]

         He never seemed to avail himself of my sympathy other
         than by mere companionship.              --W. Irving
   [1913 Webster]

WordNet (r) 3.0 (2006):

companionship
    n 1: the state of being with someone; "he missed their company";
         "he enjoyed the society of his friends" [syn: company,
         companionship, fellowship, society]

Moby Thesaurus II by Grady Ward, 1.0:

34 Moby Thesaurus words for "companionship":
   affiliation, amity, association, boon companionship, brotherhood,
   brotherliness, camaraderie, chumship, colleagueship, community,
   community of interest, company, comradeship, confraternity,
   consociation, consortship, cooperation, esprit de corps,
   fellowship, fraternalism, fraternity, fraternization, freemasonry,
   friendship, membership, partaking, participation, partnership,
   sharing, sisterhood, sisterliness, society, sodality, sorority