Group write premission was not being allowed with the default samba server configuration. Very frustrating, I tried a zillion things with chmod and acl on the client mounted folder but nothing worked. Thanks to http://askubuntu.com/questions/210808/set-umask-set-permissions-and-set-acl-but-samba-isnt-using-those I fixed it.
Adding the following to the /etc/samba/smb.conf file for the folder I wanted shared solved the problem
create mask = 664 force create mode = 664 security mask = 664 force security mode = 664 directory mask = 2775 force directory mode = 2775 directory security mask = 2775 force directory security mode = 2775
So, as an example, I had the section:
[Office-Files] path = /media/Office-Files guest ok = yes writeable = yes create mask = 664 force create mode = 664 security mask = 664 force security mode = 664 directory mask = 2775 force directory mode = 2775 directory security mask = 2775 force directory security mode = 2775
Once smb.conf is change you need to restart samba
sudo service samba restart