<form>
    <div class="form-group">
        <label class="control-label" for="lastname">multiple:</label>
        <div class="input-group">
             <select data-placeholder="Add Invitees" name="invitees[]" multiple class="chosen-select" tabindex="8">
                <option value="black">Black</option>
                <option value="blue">Blue</option>
                <option value="green">Green</option>
                <option value="orange">Orange</option>
                <option value="red">Red</option>
                <option value="yellow">Yellow</option>
                <option value="white">White</option>
            </select>
        </div>
    </div>

        <button type="submit" class="btn btn-primary">Submit</button>
</form>

$('form').validate({
    rules: {
        invitees: {
            minlength: 2,
            required: true
        }
    },
    highlight: function(element) {
        $(element).closest('.form-group').addClass('has-error');
    },
    unhighlight: function(element) {
        $(element).closest('.form-group').removeClass('has-error');
    },
    errorElement: 'span',
    errorClass: 'help-block',
    errorPlacement: function(error, element) {
        if(element.parent('.input-group').length) {
            error.insertAfter(element.parent());
        } else {
            error.insertAfter(element);
        }
    }
});