Right click on the track name. The resulting menu gives you the option of add to or remove the track from the groups.
Just like selecting inputs and outputs, hovering over "Add to group" gives you a drop down list with all the groups. Click the group and bam, it's added.
Hover over over "Remove from group" gives you a list of all the groups the track is in. Click the group and bam, it's not in that group anymore.
This is really simple and will save tons of time as it turns a five click process into only two.