ADD A TAB TO A PAGE: Silverstripe. This code will add a tab to an individual page and allow you to populate it with entries. This gist consists of two different php files.

AddTabToPage_SS.php

<?php 

  /** Partners.php
	* Partners Page.
	* This is all the logic for the Partners page.
	*/
	class PartnersPage extends Page
	{
		//Set relationship
		public static $has_many = array(
  			'Partners' => 'Partner'
 		);

 		public function getCMSFields() {
  			$fields = parent::getCMSFields();
  			$gridFieldConfig = GridFieldConfig::create()->addComponents(
   			new GridFieldToolbarHeader(),
   			new GridFieldAddNewButton('toolbar-header-right'),
   			new GridFieldSortableHeader(),
   			new GridFieldDataColumns(),
   			new GridFieldPaginator(20),
   			new GridFieldEditButton(),
   			new GridFieldDeleteAction(),
   			new GridFieldDetailForm()
  			);
  			$gridField = new GridField("Partners", "Some Partners", $this->Partners(), $gridFieldConfig);
  			$fields->addFieldToTab("Root.Partners", $gridField);
  			return $fields;
 		}


	}

	/**
	*  Partners Page Controller
	*/
	class PartnersPage_Controller extends Page_Controller
	{
		
	
		
	}

?>

<?php

//Partner.php
//This sets the settings for partners to be added to the CMS

class Partner extends DataObject
{
 public static $db = array(
  'Title' => 'Varchar(255)',
  'Summary' => 'HTMLText'
 );

 public static $has_one = array(
  'PartnersPage' => 'PartnersPage'
 );

 public static $summary_fields = array(
  'Title' => 'Title',
  'Summary' => 'Summary'
 );

 public function getCMSFields_forPopup(){

  return new FieldList(
   new TextField('Title', 'Title'),
   new HTMLEditorField('Summary', 'Brief Summary')
  );
 }

}

?>