create mask = 0775 directory mask = 0775
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
aclmount option. When default ACL is set for a directory, new files and directories created inherit the default ACL. See the answer for how to inherit group permissions in Linux for details.