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If the content gets indexed, then it's no longer protected for others to see. They could just search for the rest in Google. There's no point of blocking users and allowing Google to index it. Instead, you could build pages with excepts of the content, so Google AND users can see that, then users can decide to proceed as a logged in user or not. Google offers site authentication but only for AdSense purposes. You can give them the post details they will need to access that content, but that won't change the indexing for search. Hope that helps!