Samba tends to force its own idea of permissions and ownerships on files so that it more closely mimics the (hierarchical) NTFS style of permissions.
If you extend your Samba definition to include the force group directive you can ensure that all files created on the share (through Samba) will have the specified group ownership.
[MEDIA] read only = no locking = yes path = /mnt/local/int001/MEDIA guest ok = yes create mask = 0664 directory mask = 0775 force group = SharedFiles Your guest ok seems strange. This suggests you don't want to worry about authentication for the share, so there's no (technical) way to ensure only Bob or Sue can create files there. On the other hand, if only Bob and Sue exist then it's probably irrelevant...