This is the bit I see as the problem. How could you make sure they follow instructions? The worst would be if they left it full to fester and just went home. It’s a bit like asking g guests to clean a barbecue after use...???If we provided an empty scandanvian wood hot tub (wood fired) and said if guests want to use it, it is their responsibility to fill it up and then drain it off at the end of their usage, leaving the water no longer than 3 days (or something) then effectively they are using it at their own risk, like an outdoor bathtub.
(Sorry to appear negative, Maybe your guests would be ok)