You can use [`create mask`](https://www.samba.org/samba/docs/man/manpages-3/smb.conf.5.html#CREATEMASK) and [`directory mask`](https://www.samba.org/samba/docs/man/manpages-3/smb.conf.5.html#DIRECTORYMASK) configuration option to configure more permissive umask to allow group members (or everyone) to modify files.

For example:

    create mask = 0775
    directory mask = 0775 

There are also respective [`force crate mode`](https://www.samba.org/samba/docs/man/manpages-3/smb.conf.5.html#FORCECREATEMODE) and [`force directory mode`](https://www.samba.org/samba/docs/man/manpages-3/smb.conf.5.html#FORCEDIRECTORYMODE) options. When used the respective permissions are always added to the files.