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 creates the file/directory automatically owner group will be same as a parent.

chmod g+s /data

Verify using: ls -ld /data

You will get: drwxrws---