solved with my-fn

gistfile1.clj

(defn my-fn [result {:keys [name employer]}]
  (update-in result [employer] conj name))

(reduce my-fn {}
        [{:name "jay fields", :current-city "new york", :employer "drw.com"}
         {:name "john dydo", :current-city "new york", :employer "drw.com"}
         {:name "mike ward", :current-city "chicago", :employer "drw.com"}
         {:name "chris george", :current-city "new york", :employer "thoughtworks.com"}])