Usually the members of caucus, but a straw poll is a non binding vote and the name probably comes from the non durability of straw itself. Remember the three little pigs? The straw house only took one huff to blow down.
I think in Ohio--people had to pay to vote in the straw poll. I know they don't really mean that much. Candidates can opt out in participating in them. Like Romney opted out of participating in the straw poll in Ohio.