Mane 6 sing Sia again. I chose the three ponies that could fly to sing the melody part. Sorry Applejack, Pinkie and Rarity.

Applejack - Alto 2 (Lowest part)
*Bass line
Twilight Sparkle - Alto 1 (Second Lowest Part)
*Melody for Verse 1 part 1, prechorus, post chorus and outtro + bottom line in chorus
Rainbow Dash - Soprano 2 B (Third Lowest Part)
*Melody for Verse 1 part 2, prechorus, post chorus and outtro + middle line in chorus
Pinkie Pie - Soprano 2 A (Third Highest Part)
*Percussion
Rarity - Soprano 1 B (Second Highest Part)
*doos and ahhs
And Fluttershy - Soprano 1 A (Highest Part)
*Melody/High Melody line