Can I use Garageband on Mac to add harmony audio to a video?
If you're singing on the new track(s) you've added, definitely!  I've done it myself. You can even pan the new track(s) left or right to give your song more "space".

If you were thinking you'd duplicate your audio track in Garageband, and change the pitch of the new track, it's harder. Your harmonies probably won't stay the same number of half steps away from the main melody, so you'll have to adjust the pitch change on most notes. (As far as I know.)