If set up the pogo on your account at your home then take it to his house and plug it in. You will instantly have access to anything he plugs into it as it will be "your" device. If you just plug it in at his house and use his account to regiseter it he will have to share the drive with you first.
Of course you can also just log him out the pogoplug software on his pc and log in with your account, register the pogo, then log yourself back out and him back in to achieve the same affect. You can of course just use the same account for both of you and eliminte this problem altogether :-)