In the collaboration mode, users cannot modify the script, nor the original QVW document. However, they can create "persistent shared objects" that will exist as long as the file.QVW.SHARED exists. If you are going to enable collaboration, make sure you backup not onlye the QVW files but the SHARED files as well.
The same applies to reports and objects: users can move or change what they see, but they cannot save these changes in regards to the original document. What they can do is create a new Sheet, and create new objects within this sheet, and share this to the other users. They cannot create new dimension, because they cannot access to the script (well, they can create Calculated Dimensions, but not add new fields), but they can create as many new expressions as they need.
These objects, variables, expressions and sheets will be kept until the .SHARED file is deleted or the document removed.
Hope that makes sense.