Apple uses the ID3v2 frame "TIT1 / Content group description" for its Grouping field. This ID3v2 field is not often used outside iTunes. I don't think it is supported in MC yet.
It is quite possible that JRiver adds support for TIT1. iTunes doesn't allow custom fields and often people use this "Grouping" field for additional information. Recently JRiver added support for several proprietary Music Match tags after that was requested here.
I see you have made a couple of excellent program choices - MC and
Mp3tag. The freeware MP3tag editor is a good companion for MC because it can convert foreign tags to MC's native format.
For now, you can use the following procedure as a workaround. Mp3tag v. 2.36 or newer is needed.
1. Import the files to MC.
2. Make this new library field in "Options > Library > Library Fields > Add...":
Remember to enable the "Store in file tags (when possible)" option.
3. Add this new field to a view scheme:
4. Fill something in the "Content Group" column. This will make MC to write the tag to the files.
5. Close MC and open Mp3tag. Go to its options and configure MP3 tag writing as follows (the ISO-8859-1 tag format is needed for compatibility with MC) :
6. Load the MP3 files to Mp3tag and check that it can see both tags (iTunes'
CONTENTGROUP and MC's
COMMENT MEDIA JUKEBOX: CONTENT GROUP).
7. Make a new Action for copying the
CONTENTGROUP tag values to the
COMMENT MEDIA JUKEBOX: CONTENT GROUP tag:
8. Run the action on the files
9. Close Mp3tag and open MC. Use the library tool "Update Library (from tags)" for getting the info inside MC's library.