<label class="col-sm-2">
      Group Select
    </label>

    <div class="col-sm-10">
      <select multiple="multiple">
        <optgroup label="Group 1">
          <option value="1">Option 1</option>
          <option value="2">Option 2</option>
          <option value="3">Option 3</option>
        </optgroup>
        <optgroup label="Group 2">
          <option value="4">Option 4</option>
          <option value="5">Option 5</option>
          <option value="6">Option 6</option>
        </optgroup>
     