Hiding your content to the users will definitely hide it from the SE's as well.
Experts-Exchange tries to get around this situation by posting the actual content deep deep down in the page, where users are less likely to look. This will deter a certain percentage of people, but there is no real solution.
PS> I'm not sure what you meant, but you won't be receiving any penalties. It's just that there will be no content for the SE's to index. If you do some sort of cloaking (i.e. show the content only to SE bots) you would get a heavy penalty/ban.
I assume you would have enough members and that they join via word of mouth. No way would i bother signing up for a forum if i cant see the infomation. But from an SEO standpoint probably not the best road to go down.