To add tracks to a Listed track group, go to Manage > Track Groups and double-click the track group you want to edit. In the Edit Listed Track Group window, you'll see two lists: the track list on the left showing tracks not in the group, and the track group track list on the right showing tracks already in the group. Select the tracks you want to add from the left list and click the > button to move them to the track group. You can use the search box at the top to filter tracks by artist or title to find specific tracks more easily. Click OK when finished to save your changes.