I recently added 'Grouping' as a column to allow me to distinguish Music from Spoken Word files. MP3Tag lets me add the column but does not allow it to be entered. I can access it in iTunes if I add the file, but it seems 'kludgy'.
Could you please show us a screenshot of your "Grouping" definition in Mp3tag?
Do you use MP3 or MP4 or another music format?
Please right click on your "Grouping" column header in the File List and click on "Customize columns".
Then create a screenshot and show us your field content, especially for Value and Field.
Does it look like this:
Then repeat it for the Tag Panel "Grouping":
Right click on an empty space in the Tag Panel and click on "Customize".
Double-Click the "Grouping" entry on the right side.
How does the definition for your current Field "Grouping" (showing Music in your screenshot above) looks like?
It is not the same.
You have only defined the "Name" of the column, not what you want to see (Value) and what you want to change (Field).
Please try it with the content %grouping% in both fields.
You can read the details about the column definition in the documentation.
a look at the documentation might have helped:
Here is what shows up in the Panel. I'm not sure if that was what you wanted to see.
I did make the change as suggested and it now works. Thank you so much!
I obviously need to spend some more time with the documentation. 
