People getting in and out during the length of an evening is a clear call to keep a bit of chlorine in the tub. Add it discreetly (sneak it in a pill bottle or some similar plastic container) but add it!
And yes, all kidding aside, if you can get your guests to rinse off you will save money on chlorine, and be safer. Here in this beach community many houses have outdoor showers. They are put in primarily to rinse off sand and/or sunscreen after a visit to the beach, but they are great ways to allow a guest to enjoy a quick rinse before entering the tub.