On a fresh installation of Ubuntu 14.04.3 after sharing a folder from nautilus and not allowing guest access, accessing the share from windows (or Ubuntu itself!) results in a permission denied error. Worth mentioning that shares which guest access have been enabled for are accessible and working flawlessly.
It appears that in order to allow a user to access the shares the following command must be performed first:
sudo smbpasswd -a username
You will then be asked to enter a new password and afterwards you can access the share successfully from any other Windows/Ubuntu PC.