CIFS and AD User authentication

Currently Freenas version is 9.1.1 (only 4gb ram) and part of an AD enviroment.

Everything is working and has been for some time so don't wish to upgrade untill I can get hardware with at least 8gb of ram.

I would like to beef up security of the CIFS UFS share using AD user accounts but all ways I have tried thus far have resulted in getting a windows error "You dont have permissions to access ......."

Any pointers would be appreciated.

TYIA