The Ning that I joined belongs to my district at http://friscoisdlibraries.ning.com/. I took a few mintues to get a aquainted with the actual sight. I find it very similar to a wiki or a blog. What I do like about Ning that is different from the wiki or blog is that there is a discussion feature that is similar to Instant Messaging each other, but instead with a big group.
Another group that I joined is Sarah Dessen's http://sarah-land.ning.com/. I love her books and I enjoyed getting to see it. (I also enjoying putting my name in for some free books!