I am not a lawyer but..
Many companies do this with their employees - many employees put in, say $5 a week and their names are on the list where any significant winnings are split amongst the people participating. Any low amounts and it's put back into the pool to buy more tickets. But - they also have transparency. They purchase the tickets and then copy them all and post them BEFORE the drawing. I think there was a grocerey store in Brick, NJ (Or Toms River?) who did the pool and won a huge jackpot (each employee got over a million) sometime between 2000 and 2003.
I would think it is legal if 100% of the money coming in goes towards the pool and you don't take any cuts larger than you're fair share - winnings too small to distribute go back into the pool to buy more tickets. The only issue may be transparency - record how much comes in and then purchase, scan, and post the tickets before the drawing.
If it's done with the people are physically there it might be easier than if it's all over the internet plus then some states might claim that you're running a gambling site (yeah, even if that state has their own lotteries and/or Powerball) so you might want to look out for that, too (maybe with a nice disclaimer, only do this if it is allowed in the state you are currently living in and where any winnings might be delivered to).