Full Modal Example

View you want to call modal at:
/* Some styles for optimal form view */

<style>
#myModal.modal-body{
    max-height: 650px;
    overflow-y: auto;
}
#myModal{
    top: 50px;
}
</style>

/*End of styles*/

/* Start of Modal code */
<?php $this->beginWidget('bootstrap.widgets.TbModal', array('id'=>'myModal',
        'htmlOptions'=>array(
            'aria-hidden'=>true,
            'role'=>'dialog',
            ))); ?>
<div class="modal-header">
    <a class="close" data-dismiss="modal">&times;</a>
    <h4><?php Yii::t('svsite','Register') ?></h4>
</div>
<div class="modal-body">
    </div>
<div class="modal-footer">
    <?php $this->widget('bootstrap.widgets.TbButton', array(
        'type'=>'primary',
        'label'=>Yii::t('svsite','Register'),
        'url'=>'#',
        'htmlOptions'=>array('onclick' => '$("#registration-form").submit()'),
        'ajaxOptions'=>array(
            'type'=>'POST',
        )
    )); ?>
    <?php $this->widget('bootstrap.widgets.TbButton', array(
        'label'=>'Close',
        'url'=>'#',
        'htmlOptions'=>array('data-dismiss'=>'modal'),
    )); ?>
</div>
<?php $this->endWidget(); ?>
/* End of Modal code */

/* Button that invokes Modal */
<?php $this->widget('bootstrap.widgets.TbButton',
        array('label'=>'Click me',
        'ajaxOptions'=>array(
            'type'=>'post',
            'dataType'=>'json',
            'url'=>array(controller/register),
            'success'=>'function(data){
                $("#myModal .modal-header h4").text( data.header);
                $("#myModal  .modal-body").html(data.body);
            }'),
            'type'=>'primary',
            'htmlOptions'=>array(
                'data-toggle'=>'modal',
                'data-target'=>'#myModal',
                'id' => 'open-modal-'.uniqid(),
            ),
        )); ?>
/* End of buton */
Form view file:
<?php
$form=$this->beginWidget('bootstrap.widgets.TbActiveForm',array(
    'id'=>'registration-form',
        'action'=>array('site/register'),
    'enableAjaxValidation'=>true,
        'clientOptions'=>array(
            'validateOnSubmit'=>true, //Validation options
            'validateOnChange'=>true,
            'validateOnType'=>true,
        ),
)); ?>
    <p class="help-block">Fields with <span class="required">*</span> are required.</p>

    <?php echo $form->errorSummary($model); ?>

    <?php echo $form->textFieldRow($model,'name',array('class'=>'span5','maxlength'=>255, 'labelOptions'=>array('label'=>false), 'placeholder'=>'Enter your name')); ?>

    <?php echo $form->textFieldRow($model,'surname',array('class'=>'span5','maxlength'=>255, 'placeholder'=>'surname')); ?>

    <?php echo $form->textFieldRow($model,'email',array('class'=>'span5','maxlength'=>255)); ?>

    <?php echo $form->passwordFieldRow($model,'password',array('class'=>'span5','maxlength'=>255)); ?>

    <?php echo $form->passwordFieldRow($model,'password_repeat',array('class'=>'span5','maxlength'=>255)); ?>

    <?php echo $form->textFieldRow($model,'city',array('class'=>'span5','maxlength'=>255)); ?>

        <?php echo $form->captchaRow($model, 'captcha', array('showRefreshButton' => true)  ); ?>

<?php $this->endWidget(); ?>

Model of the form:

class User extends CActiveRecord
{

    public $password_repeat;
    private $salt = $this->email;
    public $captcha;
    /**
     * @return string the associated database table name
     */
    public function tableName()
    {
        return 'base_user';
    }

    /**
     * @return array validation rules for model attributes.
     */
    public function rules()
    {
        // NOTE: you should only define rules for those attributes that
        // will receive user inputs.
        return array(
            array('email, password, password_repeat, captcha', 'required'),
                        array('email', 'unique'),
                        array('email', 'email'),
            array('name, surname, email, password', 'length', 'max'=>255),
                        array('captcha', 'captcha'),
                        array('password_repeat', 'safe'),
                        array('password', 'compare', 'compareAttribute' => 'password_repeat'),
            array('id, name, surname, email, password, city', 'safe', 'on'=>'search'),
        );
    }

    /**
     * @return array relational rules.
     */
    public function relations()
    {
        // NOTE: you may need to adjust the relation name and the related
        // class name for the relations automatically generated below.
        return array(
        );
    }

    /**
     * @return array customized attribute labels (name=>label)
     */
    public function attributeLabels()
    {
        return array(
            'id' => 'ID',
            'name' => 'Name',
            'surname' => 'Surname',
            'email' => 'E-mail',
            'password' => 'Password',
            'city' => 'City',
        );
    }

    /**
     * Retrieves a list of models based on the current search/filter conditions.
     *
     * Typical usecase:
     * - Initialize the model fields with values from filter form.
     * - Execute this method to get CActiveDataProvider instance which will filter
     * models according to data in model fields.
     * - Pass data provider to CGridView, CListView or any similar widget.
     *
     * @return CActiveDataProvider the data provider that can return the models
     * based on the search/filter conditions.
     */
    public function search()
    {
        // @todo Please modify the following code to remove attributes that should not be searched.

        $criteria=new CDbCriteria;

        $criteria->compare('id',$this->id);
        $criteria->compare('name',$this->name,true);
        $criteria->compare('surname',$this->surname,true);
        $criteria->compare('email',$this->email,true);
        $criteria->compare('password',$this->password,true);
        $criteria->compare('city',$this->city,true);

        return new CActiveDataProvider($this, array(
            'criteria'=>$criteria,
        ));
    }

    /**
     * Returns the static model of the specified AR class.
     * Please note that you should have this exact method in all your CActiveRecord descendants!
     * @param string $className active record class name.
     * @return User the static model class
     */
    public static function model($className=__CLASS__)
    {
        return parent::model($className);
    }

        protected function  beforeSave()
        {
            $this->password = sha1($this->salt . $this->password);
            $this->password_repeat = sha1($this->salt . $this->password_repeat);
            return parent::beforeSave();
        }
}

Function in the Controller:

       public function actionRegister()
    {
                $model = new User();
                $header = Yii::t('svsite','Register');
                if (Yii::app()->request->isAjaxRequest) {

                    // to avoid jQuery and other core scripts from loading when the fourth parameter of renderPartial() is TRUE.
                    // this is useful if you want another ajaxButton in the modal or anything with scripts.
                    // http://www.yiiframework.com/forum/index.php/topic/5455-creating-ajax-elements-from-inside-ajax-request/page__p__30114#entry30114
                    Yii::app()->clientscript->scriptMap['jquery.js'] = false;

                     $this->performAjaxValidation($model);

                    if(isset($_POST['User']))
                    {
                            $model->attributes=$_POST['User'];


                        if($model->validate()) {
                            if($model->save()) {
                                $header = 'Registration was successful';
                            }
                        }
                    }

                    $body = $this->renderPartial('register', 'model'=>$model), true, true); // processOutput

                    echo CJSON::encode(array(
                        // this will be used in the Modal header
                        'header' => $header,

                        // this will be used in the Modal body
                        'body' => $body,
                    ));
                    exit;
                }
                else
                    throw new CHttpException('403', 'Forbidden access.');
    }