function registerAction($data,$form) { //you call this function when user submit registation form (or after you check payment)
      $member=DataObject::get_one('Member',"Email = '" . mysql_real_escape_string($data['Email']) . "'"); //check if the email is in the DB already
      if(!$member) { // if email is not in DB create a new member
         $member = new Member();
         $form->saveInto($member); //insert new member value from the form
         $member->RegistrationHash = md5(uniqid(rand(), true));
         $member->write(); //save new member
      }
$member->Groups()->add('myGroup');

function getCMSFields() {
       $fields = parent::getCMSFields();
$fields->addFieldToTab("Root.Content.Main", new CheckboxField('MemberOnly', 'For member only?'), 'Content');
...

function canView($member = null) {   // this function gets called when you load the page.
      if($this->MemberOnly) {
         $member = Member::currentUser();
         if (!$member) return false;
      }
      
      return parent::canView($member);
   }