In Share a simple and effective way to modify existing pages is to:
A. Make a copy of the existing page, make your changes and redeploy the page to {extensionRoot}/alfresco/extensions/share
B. Use SurfBug to determine the location of your new page component and then create a WebScript to provide the needed functionality.
C. Use FireBug to determine the location of the new page component, then re-write the underlying WebScript using FreeMarker.
D. Download the Slingshot application from SourceForge and change the appropriate page, then rebuild the share.war file and deploy.