Are you in a 2 bedroom? What is the room capacity 6? 8? - How many of you are actually staying and named on the reservation? How many nights will you be there?
If you are within your room capacity, add them to your reservation. Do they have a reservation at another Disney resort hotel? If they do, change their name up a bit on your reservation so there won't be any issues/screw ups with
MDE, etc. They can get a room key (the actual card) to show the lifeguard. Not breaking any rules if within capacity.
If there is a charge for each guest added, you will have to find out - additional food/drink for people in Concierge for the number of nights might be a possibility so not sure it's worth it. If you are staying for a few nights, I would just pay it and let them join you for the day and take advantage of the offerings too - you paid for it.
If adding them is not option, it does not hurt to ask the front desk or someone you might be dealing with at the Beach club resort/Concierge. I am not so sure the TA can help you. They might not always have the answer.
I know its not the same, but there is always the quiet pool and they can also enjoy a drink at Hurricane's. Good luck and Let us know how you make out.