To choose or elect in concert with another. [1913 Webster]
"Each of the hundred was to coöpt three others."
[1913 Webster]
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]
To assimilate (a smaller group) into a larger group. [PJC]
To persuade an opponent to join one's own side. [PJC]
To appoint summarily (with or without the appointee's consent). [WordNet 1.5]
To appropriate (something rightly belonging to another) as one's own; to preempt; as, to co-opt someone's name. [PJC]