The Collaborative International Dictionary of English v.0.48:

Companionable \Com*pan"ion*a*ble\, a.
   Fitted to be a companion; fit for good fellowship; agreeable;
   sociable. "Each companionable guest." --Mallett.
   "Companionable wit." --Clarendon. --
   Com*pan"ion*a*ble*ness, n. -- Com*pan"ion*a*bly, adv.
   [1913 Webster]

WordNet (r) 3.0 (2006):

companionableness
    n 1: suitability to be a companion [syn: companionability,
         companionableness]