[Samba] Problem: 'force user' + 'force group' don't work fine?

[Orlandini, Max]

I'm newbe on Linux and I have this problem for you:

Samba 3.0.14a (but 3.0.20pre1 also) work fine with AD users
authentication on linux (Fedora 3) machine.

I have on the root (/) the directory 'test' with permission dr-xrwx---,
owner 'pippo' group 'smbusers'

I share this directory via Samba; on the smb.conf all logged on user are
force to 'pippo' user with 'force user = pippo' parameter and 'smbusers'
group with 'force group = smbusers' parameter. The share are writable
('writable = yes').

So, I expect that: when a user make logon his identity are change on
user 'pippo' with primary group 'smbusers', so as his have
read/write/execute access to the share because are member of the group
'smbusers'...

Unfortunately isn't so... I have read many documents about this
arguments but I don't understand. If I work with 'force user' parameters
or 'force group' separately I obtain what I expect but jointly don't
work fine...

Many thanks you for your attention.

Max

P.S.: my apologies for my poor English.