@tiggerdad - I totally agree that it would be difficult. I don't see the Magic Band as being the solution either, but it could help alleviate issues.
As far as a friend visiting and possibly using the pool, personally, I think that's ok and SHOULD be allowed. In my opinion, however, that is different than going to another resorts pool just to use the pool (or a local coming in to just use the pool for free).
A pool at a resort/hotel is typically for the convenience/enjoyment of the guests. If I have a visitor, part of my "Convenience/enjoyment" is that I can spend time with my visitor, which may or may not include things like the pool, shopping, restaurants, etc..
Reporting also seems to be about as good as the magic band idea. It may or may not work. Additionally, it may or may not be an issue depending on other factors too. Is the pool otherwise busy? If there are only a handful of people in the pool, then probably not a concern. If however it's packed, I think the MB idea could work as it can allow for spot checking as well.
It's probably one of those things that they need to try and balance their rules and making customers happy with the cost/effort of enforcement.