Rental income is taxable income and has to be declared to the tax authorities where you are tax resident. Failure so to do is a criminal offence. 🙁
If you reread my reply and post you’ll see the question was regarding what tax would be payable for friends and family using the property. If no rent is being charged, no tax is payable beyond that of the standard ‘non-resident’ property tax of owning a second property.