If someone is co-opted into a group, they are asked by that group to become a member, rather than joining or being elected in the normal way. He was co-opted into the Labour Government of 1964 He's been authorised to co-opt anyone he wants to join him
If a group or political party co-opts a slogan or policy, they take it, often from another group or political party, and use it themselves. He co-opted many nationalist slogans and cultivated a populist image
neutralize or win over through assimilation into an established group; "We co-opted the independent minority tribes by pulling them into the Northern Alliance