Dear friends - DNS-323 users - I found the ONLY way to solve all the problems & wishes regarding this NAS device is to buy/use some other producer's NAS. The DNS-323 is simple a toy for home "gaming" and is absolute not to be used in any serious matter.
Folders security/share rules ...
total c_r_a_p !!
Users, groups ...
god save me from such a simplicity.
Firmware
bugs ... if I've got a nickle for each one...
Just for example, take a look to
Buffalo products (low cost Link Station, not to mention the higher/better models) and you'll see entirely new word. And best of all, my almost 3 years old Link Station Pro II works fine as the first day (with over 30 users, where several users are members of 3-4 groups + their own single user's rights). And this is not all - the same logic is dealing with folders (& subfolders).
Miracle ?? NO - just well done firmware. And as I know, this is not something extraordinary. It is just a normal thing, something a common NAS user is expecting to get.
At the end -
support 
Yes, a member of US support team tried to help me and he really tried very hard. But, how can any support effort be efficient if there is no solution ?!? And support from my nearest provider (Italy) - after 4 days they told me just "No way & look at the manual" and that's it (like I can't read or I'm too stupid to understand pictures with a few words in manual -
for such a complex & full optional device); Great Britany - even less (or better - nothing).
Well, in short, I decided to throw away this useless piece of whatever.