It's an old trick. If you're the one that buys fifteen
five dollar tickets with the groups money, all you have
to do is add another five and purchase one more ticket.
That way, if there is a winner among them, you can take
it for yourself and show the group the fifteen losing
tickets that their money purchased.
I agree with larry about checking the timestamp. If the
winning ticket wasn't the first or last ticket purchased,
you have proof positive that the winner was part of the
pool money and it belongs to the group.