So sadly there isn't any information about Ning 3.0 Sitemaps and the sitemap.xml route does not work in Google Webmasters so our only solution I could find at this moment is adding sitemaps through RSS feeds. I posted a Sitemap Hack a few years back which is basically what we will be doing here. Here is how to do it.
The Logic Behind this: Sitemaps are XML. RSS feeds are XML RSS Feeds can be Sitemaps
Step One:Go to http://ctrlq.org/rss/ and enter site:YOURNETWORK.ning.com to pull all recognized RSS feeds from your site (your site must be public to do this)
Step Two: Right-click on RSS FEED link and paste everything after the siteurl to a note document like so
You'll get URLS similar to these:
Step Three: Paste the parts of the URL after / to Webmasters
Known Feeds which should work on all Ning sites...add these to your sitemaps page in webmasters: