you need to do this

smbadduser linuxusername:ntusername


I don't think I would use root for this but As long as the user has permission to access the folder it will allow them


[Backups]
comment = Backup Folder
path = /var/www/backup
valid users = david
public = no
writable = yes
printable = no

smbadduser david:david


root# mkdir /var/www/backup

root# chown root:david /var/www/backup
root# chmod 770 /var/www/backup


or if you have a few users that need a folder to share in, setup a group and add each user to it, then set the permissions for the group on a folder