yii2 tutorial "Working with Forms" -- add field

I am new to Yii2 and tried to work out the tutorials first.

I simply tried to add a new field (name: group) to the "Working with Forms" Guide referring to this webpage: http://ift.tt/1pvE0Gd.

But user input from the added field doesn´t appear in the model.

The model:

class EntryForm extends Model
{
    public $name;
    public $email;
    public $group;  // additional attribute !!!!!!!!!!!

    public function rules()
    {
        return [
            [['name', 'email'], 'required'],
            ['email', 'email'],
        ];      
    }   
}

The Site Controller stays without any changes to the guide.

Entry-Form:

<?php $form = ActiveForm::begin(); ?>
    <?= $form->field($model, 'name')->label('Your Name') ?>
    <?= $form->field($model, 'email')->label('Your E-Mail address') ?>
    <?= $form->field($model, 'group')->label('Your Group name') ?>      <!-- additional input !!!!!!!!!!! ->

    <div class="form-group">
        <?= Html::submitButton('Submit', ['class' => 'btn btn-primary']) ?>
    </div>
<?php ActiveForm::end(); ?>

Entry Confirm:

...
<ul>
    <li><label>Name</label>: <?= Html::encode($model->name) ?></li>
    <li><label>Email</label>: <?= Html::encode($model->email) ?></li>
    <li><label>Group</label>: <?= Html::encode($model->group) ?></li>       <!--additional attribute !!!!!!!!!!! ->
</ul>
...

Notes:

Except the additional field "group", everything works fine.

User input in the "group" field appears in $_POST as expected.

But it does not appears in the model if I call $model->getAttributes()

I already checked the documentation of model->load() but couldn`t find any hint.

Thanks a lot for any help.