Play the Sun/Moon Demo
AFTER you finish the fight with skull admin aka "Big Sis"the demo will be over, boot it up again and you'll be in the Pokè Center.
Talk to the Doc. aka "I go around showing my abs in a game for kids" and he'll give you the option to transfer Greninja.
Source: I've done this this morning
EDIT: if you need more info for all the things you can find in the Demo, take a look here.