user interactivity

I want to create blog posts bu i would also want to allow registered member to add information to the existing post. They can't delete or change the original content that i have made. They can add and once they submit it, it wont display until i approve it. How would i go about achieving something like this. Basically keep adding content to existing content, but not allow previous content to be deleted/changed. Im new to drupal and would like some ideas of what can be used and if its even possible. thank you