Do you have an open blog? Then you know the importance of creating a blogging community for your blog so that contributors come & write regularly. One way to do this is to increase interaction among the members. If they can interact directly with each other from your blog, then they can use it as a platform to meet fellow bloggers with same interest & communicate for views on their own blogs or link exchange. That means more visit from your members. Besides c0-authors can communicate better to find what topics to write to make the posts coherent in your blog.
Email can be a good place for this. But it is certainly not direct from your blog. What about having private messaging system available in your WordPress blog admin? I have searched for a free plugin for that. After finding a couple of plugins that you have to pay for, I have found WP Private Messages plugin. It is free. And after you install it, registered users of a WordPress blog (except Subscribers) can send pm to each other. You will find the option under Users> Private messages. You have all the options of private messaging like saving sent messages or replying to messages etc. I have tested it with WordPress 2.8 personally. And it is working fine with the latest WordPress.