I have incredibly tiny wrists and hands and I couldn't slide the wristband off. They put it on for you and they make it as tight as possible so you can't get it off, or lose it.
If you were able to get the wristband off your selected friend's wrist and onto yours, you'd have to have almost identical wrist sizes or they would be able to tell it's been tampered with, I'd imagine.
@ Siddarth, yeah you get a number/Pokemon and they draw 128 from the total entries. If you aren't drawn, you can't battle. So if someone who has no clue what they're doing gets drawn and you don't, tough luck.
Nobody's assured getting in except that one guy at the San Francisco tourney. I don't think there's anything you can do to boost your odds either, except entering your friends but if they all get drawn instead of other people, well, that was sort of a waste of spots.