Scales to Play Over Em(add9)
Every chord is a subset of several scales: any scale that contains all of a chord's tones fits over it. Pick a chord to see the matching scales from its own root, the major keys the chord belongs to, and full fretboard diagrams for every scale on guitar.
A Em(add9) chord contains the notes E – F# – G – B. Any scale that includes every one of those 4 tones fits over it — from the root E, that means E Natural Minor, E Melodic Minor, E Harmonic Minor, E Rock 'n' Roll, E Dorian, E Dorian Bebop and E Gypsy Minor. Start with the first scale listed: it is the most common choice, and the others color the same chord tones differently.
Em(add9) is also diatonic to 2 major keys: D major (relative minor Bm), G major (relative minor Em). In a song from one of those keys you can also solo with the key's own major scale or its relative minor pentatonic across the whole progression, not just this chord.