Hi,
Firstly since Site B is a workgroup - in order to be able to access the file Storage on a file server in the domain- Make sure you have a user account created on the domain fileserver and the same user account on any workgroup machine that is going to use the fileserver.
PS: The password for the user has to be same on all the computers
After this is taken care of you can Map a drive to the shares on the fileserver
Regards,
TG82