I'm assuming, from the way I understand your question, that you WANT the SE's to list your protected pages but, when someone actually clicks the link, they get hit with the membership sign-up. Am I right?
You could do it by cloaking your pages, so the SE's can see the content but the visitors can't. The only drawback is, the cached version would still show the content the SE's saw. Not many typical innerwebs surfers are savvy enough to look at the cached version, though.
I imagine there might be another, more complicated way to do it but, if it were me and the crowd I was trying to attract wasn't the innerwebs savvy type, I'd just cloak my shit.