Wrist bands are required, and you get as many bands as you have people staying in the room. If you have a 1 bedroom, you could put up to 5 names on the room, a studio is 4, a 2 bedroom is 9 people. They would get keys to your room which they can exchange for wrist bands on a daily basis.
Some resorts with pools here do sell day passes usually for about $50 per person per day, but the last time I checked (in January), Aulani was not one of them. They said it is something they might consider in the future after the resort is fully built if it seems like it might not negatively impact guests experience. We were sad since we have some local friends that we would have liked to have joined us and would have gladly bought day passes, but had a great stay anyways.