How do I update grouping or description for multiple items?

Question

As the question implies, I would like to update information in the Grouping and Description fields for multiple items as a way to keep things organized. What I have discovered is that there are some quirks in the new Get Info window that were not in past implementations, which are completely inexplicable and nonsensical to me.
For instance, when you open the Get Info window for an item that has no information in, for instance, the Grouping field, you will not see a Grouping input field, as in this screenshot. The Grouping field should be where the bottom arrow is. Notice also that the Description field is available. I will get back to that in a minute.
If you manually double-click into the Grouping field on the main iTunes screen, you can enter the information there. Then, when you open the Get Info window, ta-dah! The field magically reappears, as in the screenshot below.
The trouble comes when you want to edit this field for multiple tracks. If you select multiple tracks, and they do not all have the Grouping field already populated, the field does not appear. What this means is that you are forced to manually enter the information for every track! This is completely unacceptable when you need to edit information for, as in my case, 122 tracks. The following image shows the Get Info window that appears after selecting two items, one with the Grouping field manually populated, and one with no info in the Grouping field.
Not only has the Grouping field disappeared, but the Description field, as well. Thanks, Apple, but I think I can decide for myself whether or not to populate these fields for multiple items. Let's get 'em back, huh?
SO… does anyone know of any valid method for accomplishing this task? Without manually entering the same information in hundreds of fields? This is the future; computers are supposed to do these monotonous tasks for us, aren't they?
Oh, please don't suggest Option-clicking to open the older, better, Get Info window, as Apple has decided that we should not have that option any longer. We apparently have to organize our music their way. Think different!

Answer 1

The Add Field button was in the editors of 12.0.1 (where the new-style editors had a very "half-finished" feel)  - it is not there in 12.1.  Apple have determined which metadata fields should be available for each kind of media, with no option for the user to modify this.  In Apple's own words: "Get Info has been completely redesigned in iTunes 12 to focus your attention only on what’s necessary for the selected item."
The only options are:
temporarily change the media kind (Options tab) to one that includes the field you want to add values to, enter those values, and then change the media kind back again
make the relevant fields visible in an appropriate list view and edit values there - the big limitation being that you can only do this one item at a time
I can see - to a degree - why Apple may have made some limitations so that metadata elements are available based on their relevance to different media kinds.  However, one of the issues mentioned above seems like a bug - if a single audiobook file can have a Description tab, I see no reason at all for that not to be available for multiple items of the same kind.
If not already done so, report these issues and dissatisfaction with the "new" metadata editors in 12.1 (and the absence of a "back door" for the old-style ones) using Apple's iTunes feedback page.