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You can use create mask and directory mask 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 and force directory mode options. When used the respective permissions are always added to the files.

In your scenario if all your users share a common primary group, using above options would be 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 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.