Does restricting forum access hurt Search Engine rankings for forum content?

My crew stocks a client's forum with highly informative original content geared towards niche specific keywords. He's been getting a lot of targeted search traffic. However, we'd like to boost the number of REGISTERED USERS. To accomplish this, we're thinking of restricting thread/post content viewing to registered users.

My question: Since we spent a lot of time and effort crafting the titles and content to get search engine traffic for our client's keywords, will this optimization be affected if he blocks unregistered users from viewing the content?

Thanks in advance

 

If the content you speak of is in the forum, yes.

 

actually didnt google just add an option for you to add pages to be crawled that only logged in members can see? Thought I just saw that recently check it out pretty sure its there in wmtools somewhere