BuddyPress.org

You don’t have to do anything.. Just install BuddyPress on a single WordPress install and all your users are contributors by default.. This means they can submit posts easily. You can make the process much easier by using a plugin like TD Midi Forms which allows your users to submit posts from the front end and even allow them to edit their own posts. A good tutorial:

http://thedeadone.net/blog/tutorial-video-for-tdo-mini-forms/

Good luck