Advice re setting up a way for users to share Specific Posts only

Hi
Firstly have to say you guys are great - support has been top notch!
Need some advice if poss - have read lost of your different stuff on users sharing posts BUT there is a minor issue. All of my users have a clone of a main blog site where posts are automatically posted for them. so for example 100 users will all post the same thing at the same time. However they also have the ability to post their own unique articles and it is ONLY these ones that I am looking for a way to share between users and was hoping you could suggest best way to do this - I like the idea of trying to create a community of users where they can share ideas and message each other etc. Great for any assistance or just being pointed in a general direction. Many thanks