Whoever creates the file on /data make automatically owner group should be the group owner...

When user creates the file/directory, user owner will be user itself and group owner will be the primary group of the user.

There is one Special Permission SGID, when you set the SGID bit on directory. When users create the file/directory automatically owner group will be same as a parent.

1. chmod g+s /data

2. Verify using: ls -ld /data

You will get: drwxrws---