From a tax point of view, the only people allowed to stay in your apartment, without you needing to declare potential income and therefore a possible tax liability is the parents or children of the owner. All other friends or family could potentially give rise to a tax liability, even if they pay no rent to you. It's under the Turkish "low or no value" for tax income section of the Turkish tax guide. You are also allowed to earn approx 3,300tlr (each name on Tapu) without declaring anything to the tax man. Hope this helps, especially if someone maybe trying to "shop" you.