I like the "We hope you will use Ning to:" list. That should really help conference attendees figure out what the purpose of it is. I know that for a while, my thoughts about social networks were, "That's cool, but they don't seem all that useful." So that list should open the eyes of new members to why they should participate.
Still working on this. But it's best if they do, since then they will be alerted to all posts in reply. For CUE we set up a discussion for every session, but then realized the speakers were not notified in any way when someone posted about their session.
Well, the nice think about Ning is that you can set up a network in all of about 5 minutes. The NECC network is really just their standard setup with a nice header and custom colors. Don't really even need those to get started or to be effective!
For NECC, I asked the conference organizers to write the "Welcome" note, and then I started the "Introductions" forum thread. I then announced the site to my network and blogged about it, and then the NECC folks announced it to their lists.
I'm not doing any convincing to get the speakers to engage, but they are being asked to by NECC. :)