How do I configure Samba so that any new object created in a
particular Samba share always ends up with a particular group
ownership?

Example:
- I have a share /home/my_project

- Directory 'contracts' inside the share, and ownership:
# ls -l /home/my_project
drwxrwx--- 2 someuser mygroup 4096 May 11 01:48 contracts

- User 'joe' is has default group 'users', and is member of the group
'mygroup'.

- A new file 'new_file' is created by user 'joe' inside the
my_projects,
folder, and ends up with the following permissions:
# ls -l /home/my_project/contracts
-rwxr-xr-x 1 joe users 529 May 12 04:08 new_file

NOTE: the ownership and permissions on the file that 'joe' created
are
those that 'joe's' user/group and umask dictate.

- The relevant part of smb.conf is:
[my_project]
path = /home/my_project
force group = mygroup
create mask = 0660
directory mask = 0770

What am I doing wrong?

Thanks,
--Ulf