A Google AdSense post shares an old tip to AdSense publishers about how Google can return relevant ads even when there is a login required to see the ad.
The issue is, Google's ad targeting feature works by sending a Google spider to read the page. If the spider/bot cannot read the page, the ads might not be on target to the content on the page.
The way around that is to give Google's ad bot a way to read the page behind the login.
To grant the AdSense crawler access, log in to your AdSense account and click on 'Account settings.' From there, scroll down to the 'Access and authorization' section and click on 'edit logins' next to crawler logins.

Then click on 'Add a Login.' You'll be prompted to fill in pertinent information that the AdSense crawler will use to log in to your site and crawl your content.
After that, you'll need to log in to Webmaster Tools to verify your website, if you haven't already done so. Once verified, the crawler will be able to log in successfully and scan your content to deliver more targeted ads.

Again, this is not new, it is pretty old but a reminder doesn't hurt.
Forum discussion at WebmasterWorld.

Comments:
patagonia
12/12/2011 08:53 pm
Nice tip, but what can I do if I got 200.000 user in my social network. I cannot give all this user login data. No way
Tride Darsono
06/17/2012 08:11 am
just manual blogging with blogwalking and share in your social network?? By the way what about your vacation ever go to Bali, Indonesia??
Andrew Beaven
02/27/2013 08:53 am
That's what my first thought was when reading this. Thankfully I have an admin account that has access to all these hidden pages, so I'm hoping that using that username/password will solve for all.