The Collaborative International Dictionary of English v.0.48:

co-opt \co-opt"\, coopt \co*["o]pt"\, v. t. [See Cooptate. Cf.
   F. coopter.]
   To choose or elect in concert with another. [R.]
   [1913 Webster]

         Each of the hundred was to coopt three others. --Jowett
                                                  (Thucyd.).
   [1913 Webster]

   2. To choose or elect as a colleague or fellow member of a
      group; as, The church members co-opted individuals from
      similar backgrounds to replenish the congregation.
      [PJC + WordNet 1.5]

   3. To assimilate (a smaller group) into a larger group.
      [PJC]

   4. To persuade an opponent to join one's own side.
      [PJC]

   5. To appoint summarily (with or without the appointee's
      consent).
      [WordNet 1.5]

   6. To appropriate (something rightly belonging to another) as
      one's own; to preempt; as, to co-opt someone's name.
      [PJC]