Is this an epub or a DPS question?
then it is not possible within the Digital Publishing Suite. I didn't know, just asked.
Moved to the epub forum.
In my opinion, this is more a publication worlkflow issue than an epub-related one.
If it were an Adobe DPS publication, you'd just make your publication and then you would share it with a restricted set of users via the Content Viewer. As far as giving a copy of your work to somebody else goes, this is one of the best ways to share a work without spilling the beans in advance —that does not mean that a DPS 'proof' cannot be copied, of course, it just means that the copier would have a very hard time trying to do it.
However, an epub is quite different. As long as you are not using a DRM system to safeguard the content —and I am a bit skeptical about it—, you are giving away a manuscript of your work ready to go to the market —images included—.
But, as it is a normal procedure to use copies for proofreading and getting informed opinion about your work, in the stage of production you are in yet, I would use a PDF with restricted privileges —no copy, no printing and a watermark—. Not that it cannot be copied by somebody with bad intent, but... difficult to do.
If your work uses a lot of interactivite of the epub 3 format, then that option may be rather limiting, but it is up to you to ponder the advantages and disadvantages of the options you have.
Just my 2c.
Gustavo (Posting from Madrid)