You can embed Google Custom Search to allow users to search all the content on your website:
Make sure you paste the custom search embed code directly into the template of a page, not into the content editor.
Google will not index your site right away, so you should work to get some external links to your new nation from other sites or submit it to Google directly. Once Google and other search engines know your site exists, NationBuilder does provide a sitemap that will help the search engines index all of your public pages.
To learn how to set up search with the Pershing Forest theme, visit this page.