SEO is an abbreviation for Search Engine Optimization. In Layman terms, it basically means trying to make your site appear as the first result in search engines based on keywords mostly related to your site.
Making a sitemap is like asking search engines to prepare themselves to index your pages until you submit the sitemap to any search engine, in which case the indexing starts.
I assume by "invite only" you mean that only selected individuals will know the URL to sections not available to the public. If you make a sitemap, then to maintain security, don't include these URLs in the sitemap.
Other than that, I'll have to go with a no for sitemap creation unless that introductory page on your site is so spectacular that millions of people will want to see it, in which case you'd only index that one page, but even then, search engines might have already indexed it.
If by chance search engines have indexed your private pages and you still want to make them accessible to selected individuals just by typing in the URL only, then you want to add the following between
</head> section of the HTML code of each affected private page:
<meta name="robots" content="noindex">
That way, search engines will remove the affected pages off their index and eventually will no longer list them.