I'd call it key of concert Em (same signature as G), which would transpose to C#m (same signature as E) on the Alto. Make sense?
Guitar chords in the song are Em, D, A, with a C dropped in during the refrain. Would love to hear thoughts on what to play on the alto during the verse and refrain chord progressions when there is sax, but it sounds to me like he's hanging on the V, IV, or I note of the chord being played and then making little runs - like G#-E-D#-B - here and there.