People love to share information about cool sites they have found and interesting posts they have read. Taking advantage of this is a great way to bring in extra traffic. If you make it easy for visitors to your blog network to share the things they see and easy for your bloggers to promote the things they are doing, then you should get a decent amount of "free" promotion—assuming the content on your site is worthwhile, of course.
Twitter is a microblogging service that is enjoying a huge amount of success at the moment. It allows you to share the answer to the question "what are you doing?", as long as your answer is 140 characters or less! If you aren't already a member of Twitter, why not join? It's free to make an account on www.twitter.com, and even the busiest people in the world should have time to share the occasional 140 character message!
One common answer to that question is "posting a new blog entry" with a convenient link so that others can read...