I have posted this discussion before in a different wording but have expanded on it in the hope that I can get a few answers/input.
I'll offer you some input, but someone else will have to provide you the info you want. You also might do some digging. I think I've picked up hints of a lot of conversation on these topics on the Ning Creators forums/ groups.
1) Marking some pages as members only as an incentive to join.
I personally do not agree with this approach. I believe the substance of a site, the value projected in particpation on a network, and tranparency in place of conditional content are superior approaches.
2) Marking Creator and Admin profiles to private as an incentive to join.
Again, I think a key in getting people to join is delivering what they expect instead of requirements to join in order to proceed. I feel a lot of people consider that approach as an insult to their intelligence and a manipulative tactic. Not everyone obviously...many creators report great success with dangling the carrot. However, I'm not convinced. I think it may depend more on other factors, perhaps a quantitive-based network and corresponding audience vs. qualititative. Also, if you hide your best content pages and contributors, you're likely missing out on that SEO and that many more conversions from people that are impressed and want to network with you.
So you'll have to hear it from the experts I guess. I think privacy is an option best reserved for those members who value it. Privacy ccontrol is more of a standard issue on the likes of Facebook and Linkedin, etc. However, I think the open community aspect of Ning networks are a positive distinguisher.