Add to Track Groups from the Tracks List
At the moment, you need to go into a Track Group, then add tracks to it. This means if you're categorising a few hundred (or thousand) songs, you need to open each track group, then read through the whole track library, then repeat for each group.
It would be nice to have the option of doing this the other way round -- start with the list of all tracks, then add each track into one or more groups. Perhaps an "Add to Group" option in the Tracks list could do this.