In the sharing dialog (where you enter the email addresses) there is a link at the bottom that says "More options". This is where you can adjust the re-sharing options. Default is "Everyone the file is shared with can share it with others (at their own role or lower). The other option is "Only let co-authors share this file with others."
After the file has already been shared you can change the sharing options by doing the following:
- Select the file in the Acrobat.com organizer (files tab)
- Select share file (either in the lower left of the window or select Share Document from the drop down menu)
- Click on Change options at the bottom of the menu that slides up from bottom left
Co-authors have the same rights as an author and there is currently not a setting to prevent co-authors from sharing. If they do not need to have the same rights as an author, try changing them to a reviewer or reader and changing the share option to "Only let co-authors share this file with others."
You can find more information about the different roles here.
If I've misunderstood your question, please post again.
