For this series, we are adding the 9 on string 2, and including the 5th on string 1, as in the first example. These are common voicings and also ring sweetly.
Add9 means that we are adding the 9  to the chord while the 3rd of the chord is still present. For C, the 9  is D… CDEFGABCD = 123456789.
Again, these chords can be used anytime we see the symbol, if the sound fits.