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.

Depending on how your users are configured:

* If all your users share a common primary group, using above options is sufficient.

* In case the primary groups aren't shared, but users are members of common group, set the directory group to the common group and apply [*setgid bit*](https://en.wikipedia.org/wiki/Setuid#setuid_and_setgid_on_directories) to it. When *setgid bit* is set to a directory, new files created inherit the directory's group and new directories created will have the *setgid bit* set as well.

* Use [*POSIX Access Control Lists*](http://manpages.ubuntu.com/manpages/trusty/man5/acl.5.html). To enable ACL for a file system, you might need to use `acl` mount option. When default ACL is set for a directory, new files and directories created inherit the default ACL. See this answer for [how to inherit group permissions in Linux](https://unix.stackexchange.com/a/115632/239817) for details.