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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 created files and directories.

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 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. 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 for details.