VERB (4)

1. choose or elect as a fellow member or colleague;
- Example: "The church members co-opted individuals from similar backgrounds to replenish the congregation"

2. neutralize or win over through assimilation into an established group;
- Example: "We co-opted the independent minority tribes by pulling them into the Northern Alliance"

3. appoint summarily or commandeer;
- Example: "The army tried to co-opt peasants into civil defence groups"

4. take or assume for one's own use;
- Example: "He co-opted the criticism and embraced it"

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]

WordNet (r) 3.0 (2006):

co-opt
    v 1: choose or elect as a fellow member or colleague; "The
         church members co-opted individuals from similar
         backgrounds to replenish the congregation"
    2: neutralize or win over through assimilation into an
       established group; "We co-opted the independent minority
       tribes by pulling them into the Northern Alliance"
    3: appoint summarily or commandeer; "The army tried to co-opt
       peasants into civil defence groups"
    4: take or assume for one's own use; "He co-opted the criticism
       and embraced it"