[Samba] "valid user" <-> "valid userS"

[Mona Meyer]

>> It does NOT work with:
>> invalid userS = @users
>>
>> curiously it DOES work with:
>> invalid user = @users
>>
> Did you check to see if it works with both commented out?
>
> And the reason for my question is samba honors the *nix filesystem
> permissions, so if you have them set properly it will do the correct
> thing.

Ok, with both commented out it works, too. But this only means this
function does not work at all, doesn't it? Because I wanted to use the
 filesystem permissions AND the permissions granted by samba. Any
suggestion, where I can find out, what I did wrong?