Hi Peter,
See this thread where I answered a similar question. People keep asking for this ability to have multiple users use the same blog but only be able to edit their own posts, so when I get to working on the next round of blog improvements I may look into making it possible to do this. However, as stated in the other thread I still would not let just any authenticated user use the blog, that would be very dangerous, it is designed for trusted users.
Best,
Joe