I am learning guitar by myself. I see that the tab composer usually adds more notes that are not in the original sheet to make song more impressive. Please look at the notes in the circles in my picture.
Can you tell me the name of these notes and how to add them to the music sheet.
Those circled pitches are pitches played by other musical lines. In this case, the notes that you're thinking of comprise the melody, and these circled notes comprise the accompaniment.
These particular red pitches are played by the piano in the original, so that's how they're determined in this arrangement for guitar. Otherwise, if you're arranging a piece of music that doesn't have those pitches, you'll just use pitches that (usually) fit within the current chord. You'll notice, for instance, that that first circled D fits nicely into the current G-major harmony.
If those (freehand!) circled notes were not present anywhere in the original, then the tab arranger made a stylistic decision to include them. In this case, it makes a nice arpeggio figure to go with the melody. Also notice that the melody is played (notes with words under them), but the notes circled are adding to the harmony of the chord symbols. These notes could be any notes that fit in the chord, as Richard notes. The notes also sustain a relentless quarter note rhythm, never stopping to rest. It's a refreshing break from the blocks of chords and strumming patterns approach to chord-melodic playing.
As far as I understand, you mean there are notes in the arrangement which are not part of the MELODY.
In fact the notes you circled ARE in the sheet.
Each simple chord (triad) consists of 3 notes. So it's absolutely possible to add ANY of the three notes from that chord. There will be no disagreement. Your sheet says G is played in measure 9 - which means it's safe to play (add) any of the 3 notes from chord G. This implies harmony (2 or more notes played simultaneously implies harmony or lack of it). In all of your sheet the added notes do not contradict the underlying chord at any moment, so in terms of harmony there is no contradiction.
Tip: it helps to learn how chords are created. Simple triads. Then more complex chords, etc. If you learn how to understand sheet (not just tab) your understanding of theory will improve.
