{{funcionarios|json}}

HTML

<div class="form-group">
  <label>Funcionarios: </label>
  <ui-multi-select placeholder="Buscar..."
                   close-on-select-item="false"
                   ng-model="funcionarios"
                   search-options="getOptions(value)">
    <ui-multi-select-item ng-repeat="item in funcionarios"
                          ng-value="item">
      {{item.username}}
    </ui-multi-select-item>
    <ui-multi-select-option ng-repeat="option in items"
                            ng-value="option">
      {{option.username}}
    </ui-multi-select-option>
  </ui-multi-select>
</div>

JavaScript

angular.module('app', ['ngUiMultiSelect'])
.controller('ctrl', function($scope, $timeout, $http){

    $scope.getOptions = function(viewValue){
      return $http.get('https://jsonplaceholder.typicode.com/users?q='+viewValue)
        .then(function(resp){
          $scope.items = resp.data;
        })
    }

})