Normally after grouping objects one gets the ‘U’ on the list, but not this time. I’m only getting the A, Family, Link, Track, and Material. I’ve searched the manual and online.
Sounds like you accidentally pressed the letter C that was just below the letter U after you grouped the objects. Each of the objects in the group must have had a specified polarity setting. When these conditions are met it is possible to Convert the group of many objects into a single object. This is often ideal for exporting.
This action can be undone, but only within the same session in which it is enacted. If the scene is closed and reopened the action cannot be undone. The only way to undo this is with the undo button. The Undo feature remembers the15 most recent user actions,including boolean conversions.
Hope this helps.