samba - Apr 2012 - smb.conf: per group config for multiple group environment

I have 2 groups of users: sales and support.

In my samba server, I create 2 shares: sales_share and support_share.

I want to make:
  - sales_share available to sales group but not support group.
  - support_share available to support group but not sales group.

First, I try to config sales_share:

[sales_share]
        path = /mnt/sales
        valid users = @sales
        force group = sales
        read only = No
        available = No
        include = /etc/samba/%G.conf

I create file /etc/samba/sales.conf as follow:

        available = Yes

I restart smb service and try on Windows PC, login as sales user and I can
access to sales_share.  Login as support user and the sales_share is not
available.  So far so good.

I then create support_share:

[support_share]
        path = /mnt/support
        valid users = @support
        force group = support
        read only = No
        available = No
        include = /etc/samba/%G.conf

and create file /etc/samba/support.conf as follow:

        available = Yes

Restart the smb service but the outcome:

  - sales_share is available to sales and support group
  - support_share is available to sales and support group

How may I configure samba share to allow per group config if I have
multiple groups?

-- 
Best regards,
Chau Chee Yang