Ubuntu – How to get group write permission with Samba 4

I have a Samba share server running Ubuntu. After upgrading to 14.04, I had Samba upgraded from 3 to 4. Since then, I can't get group write permission on my newly created directory or files.

What was previously working in Samba3 was using these settings:

security mask = 000
force security mode = 660
directory security mask = 000
force directory security mode = 770
force user = nobody
force group = Domain Users

These settings were removed in Samba 4 (see https://wiki.samba.org/index.php/Samba_4.0_Features_added/changed#smb.conf_changes).

But now, my directories are created "drwxr-x— 2 nobody Domain Users" and my new files "-rwxr-x— 1 nobody Domain Users".

So what is the way in Samba 4 to allow my users to create and share with write permissions new directories and files ?

Here is my full samba config:

[global]
    workgroup = WORKGROUP
    server string = %h server (Samba, Ubuntu)
    interfaces = 127.0.0.0/8, eth0
    map to guest = Bad User
    obey pam restrictions = Yes
    passdb backend = ldapsam:ldap://ldap
    pam password change = Yes
    passwd program = /usr/bin/passwd %u
    passwd chat = *Enter\snew\s*\spassword:* %n\n *Retype\snew\s*\spassword:* %n\n *password\supdated\ssuccessfully* .
    syslog = 0
    log file = /var/log/samba/log.%m
    max log size = 1000
    load printers = No
    domain master = Yes
    dns proxy = No
    ldap admin dn = cn=root,dc=example,dc=com
    ldap group suffix = ou=Groups
    ldap idmap suffix = ou=Idmap
    ldap machine suffix = ou=Computers
    ldap passwd sync = yes
    ldap suffix = dc=example,dc=com
    ldap ssl = no
    ldap user suffix = ou=People
    usershare allow guests = Yes
    panic action = /usr/share/samba/panic-action %d
    idmap config * : backend = tdb

[CommonShare]
    comment = Common share
    path = /srv/samba/common
    valid users = @myusers
    read only = No
    create mask = 0660
    force create mode = 0770
    directory mask = 0770
    force directory mode = 0770
    inherit permissions = Yes
    inherit owner = Yes