function SubscribeForm() {
      $email ='';
      $newsletter_group = DataObject::get_one('Group', "ID = 4");
      $member = $this->CurrentMember();
      if($member):
         $email = $member->Email;
      endif;
      if(!$member || !$member->inGroup($newsletter_group)):
         // Create Fields
         $fields = new FieldSet(new EmailField('Email','Email', $email,'100'));
         // Create Action
         $actions = new FieldSet(new FormAction('doSubscribeForm', 'Go'));
         // Create Validators
         $validator = new RequiredFields('Email');
         //Create Form
         $form = new Form($this, 'SubscribeForm', $fields, $actions, $validator );

         return $form;
      endif;
      }

   function doSubscribeForm($data, $form){
      $email = Convert::raw2sql($data['Email']); // prevent sql injection
      $existing_member = DataObject::get_one("Member", "Email = '$email'");
      
      if($member = $this->CurrentMember()):
         // Get member with matching email.
         // If logged in user matches that member add them to newsletter group.
         if($member->ID == $existing_member->ID):
            $newsletter_group = DataObject::get_one('Group', "ID = 4");
            $existing_member->Groups()->add($newsletter_group);
         endif;
         return Director::redirectBack();
      else:
         if($existing_member):
            return Director::redirect(Director::BaseURL() . 'Security/login');
         else:
            return Director::redirect($this->Link("/sign-up/?s=1&e=" . $email));
         endif;
      endif;
      }

            <div class="SubscribeForm">
               <% if SubscribeForm %>
               $SubscribeForm
               <% else %>
               <div> You've subscribed! </div>
               <% end_if %>
            </div>

$newsletter_group = DataObject::get_one('Group', "ID = 4");