Hi Allison, starting a private online community -- that's really exciting!
In my experience, those who are most successful dedicate time to the cultivation of community. Some people think that as soon as they announce a private community their entire membership will sign up and start uploading fantastic content and connecting with on another. While that will happen, it takes time to encourage and create those connections.
One of the ways to drive recurring traffic is through powerful content. At least initially, content creation will be on you and your association. Eventually your members will be adding to it but for now, the way to get your members involved is to provide content and opportunities they can't find elsewhere. (Here are some tips for content: http://www.yourmembership.com/blog/2011/03/ten-tips-for-creative-content/ )
I could talk about community all day and my response is already pretty long, so I'll leave you with this link to 5 tips for building community: http://www.yourmembership.com/blog/2011/03/five-tips-for-a-successful-launch-of-your-online-community/
If you have any questions, feel free to connect with me. Private communities are my passion and I could spend all day talking about them. I wish you much success with your launch and am sure that you will find it to be very rewarding.
-Christina