You'll automatically get signed into Minecraft with the account you bought it with, and signing into Windows 10 with another account won't let you play that version of Minecraft UNLESS you download it again for that account. So that you don't have to buy it twice just sign into Windows 10 with THEIR account, then on the Windows store sign in with YOUR account and download it again. It means having 2 copies of Minecraft on 2 separate accounts. That's how I did it so that my niece has her own version associated to her gamertag and gamerscore.