you need to show them something to subscribe (i.e first page)
facebook page post
Your network is private, so that's going to make it more difficult to promote, and bad for search engine optimization (SEO). Help Seeker is correct in that you need to at least make your home page visible.
When I built my first Ning network five years ago, I waited months before I launched it, as I needed a strategy to build membership. If you don't have any members yet, you can take your network offline for a while, which will allow you time to develop strategies for reeling in members, keeping them engaged, and making it profitable for you (if that's what you wish).
It's not as simple as asking someone how they promote their networks, since each network has it's own niche, and hence, require strategies that will best work for their particular niche. You need to clearly define what it is your network is about, then ascertain what it is you want to achieve. Once you've done that, you can formulate your strategy.
From what little I can see, you may also want to explore learning how to customize your network, or hire someone who can customize it for you.
Make it public and moderate (approve) new members if you are trying to keep out spammers... :)
I want to view your site but I don't want to join to see what it's about. :)