That's a great question.
When a site allows search engines, an XML format sitemap is submitted to search engines so that all pages are indexed. This includes any URLs that are not discovered through the normal crawling process. A dynamic XML sitemap is automatically generated and available to Google and other search engines.
We have already created a great post on this topic here.
It's important to note that 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.
Complete and current information on how to use NationBuilder is available in the documentation section.