By default each portal user has a private folder group defined for them that contains the files and folders they create. Users can share these files and folders with other portal users making them visible to the recipients within their portal account via the User Interface, Mobile App or Agent.
However, although a user has access to other users files through collaboration, they cannot copy these files through the various interfaces directly to their own target folder as this location lies out with the owners private folder group. It is currently not possible to copy between folder groups in such a manner.
This limitation will be addressed in the next major release of the software.