Technically, it's not allowed. How you choose to handle it is up to you. I know when we were at the Contemporary one year, they came by and asked us for our resort ID (and we were just at a table near the pool). It was a spring break and so it was a busy time. That is the only time I've been asked for that.
One year, we were staying at the Beach Club, and my DH's friend that was living in Tampa came over and spent the night with us (we had 2 beds in the room, don't worry!). We all swam together and had a nice time. I didn't know at the time, that I should have told the hotel that we had more than 2 adults, and I was technically supposed to pay an extra fee for him. Honestly, I really didn't know. But no harm done, we didn't need any extra towels or anything.
I think they are more conscious of this during busy times. If you get caught breaking the rules, I think they can ask you to leave the resort.
Maybe you could all go to a water park?