you need to set an storage option like Seblod has recommended, when you add the new field. This should be: "custom" and saving in "introtext". Otherwise you will be run in troubles later on.
The GroupX field should be in custom storage and saved in introtext, that you getting no troubles when you wanna calling groupx field by template overwrites.
The fields inside groupx (Contentype of the group) - just let it like seblod recommend it.