As I see it you have two courses of action you can take.

The first is lease the pool space from your friends and rent it out as your own. Maybe enter into an arrangement for coupons with your friends so people who use your pool get a discount on food.

The second is convince them to remove the pool altogether and repurpose the room

Filling the pool with anything other than water, even foam balls, poses a significant safety risk to the users and more than likely would void any insurance policy the owners have. Keep in mind that pools are not flat surfaces but rather they have numerous drains and intakes that could cause significant injury. Modifying the pool itself would cost more than just filling it in if you are doing it to code.

Unless you plan on entering into a landlord / tenant style lease agreement with your friends I would suggest waiting until they decide what they want done with the pool before you go making any plans.