Setting up file sharing in Drupal 7
I suggest using Privatemsg module it had a nice attachment field and now it is fieldable that means you can add highly customizable file fields. 

Thou Privatemsg module will work not as content type but more like internal messaging tool I also suggest checking out Notifications which depends on Messaging module. It will send emails upon creation or edit of chose content for all users you chose. You may also look into Team Notifications module which will allow your users to create subscriptions for others. Which is integrated with Organic groups module (OG). OG is best way for grouping and allowing users to group themselves yet if you are looking for a simple solution this may not fit due to it`s extensive functionality which results in fairly complex results.