How do I define a "dropbox" type share with Samba?

Issue

• The following share definition will allow users of the gtest group to add files to the share
• The user will not be able to see the contents of the dropbox, and the admin user will be able to retrieve the files.

# mkdir /dropbox
# chown admin:dropgroup /dropbox
# chmod 770 /dropbox

The share is defined as:

[dropbox]
        path = /dropbox
        valid users = +dropgroup
        admin users = admin
        write list = +dropgroup
        force group = dropgroup
        read only = No
        create mask = 0200
        directory mask = 0770
        hide unreadable = Yes

• Copying a file to the share as user1, it will result in the following file permissions:

# ls -la /dropbox
total 8
drwxrwx---.  2 admin dropgroup   39 Oct  5 10:27 .
drwxr-xr-x. 21 root  root  4096 Oct  5 10:21 ..
--w-------.  1 user1 dropgroup    7 Oct  5 10:27 New Rich Text Document.rtf