I run a ning group that is a spin off of my yahoo group but I added sub sections for things that have to do with twitter, one was for autism twitter day and now one for autism book swap.
They are like groups with forum, photos, groups, etc so you connect with a member, can have chats for members and run contests, giveaways, etc.
The two other sites I write at have private ning groups for their editors. You can have it set up to send group messages but I prefer not to get all those messages from members trying to sell their stuff.
On twitter I joined a ning group that was promoting a $25 shell gas card to the 325th member and I timed it to win, gave the card to my sons aide for Christmas gift.
I am in another one now that is giving away a flip camera phone to one active member on Jan 1. Also another group is one for wild earth tv that we watch from twitter link to see this african bush drives and they have fireside chats CAT time and I belong to an elephant group where we discuss those issues at zoos and I rate lots of the photos, can import them from flickr easy.
you can check mine out to get an idea
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