To keep your home WiFi secure, don’t share your main WiFi password and make your guests use the guest WiFi built into most routers. This isn’t to be selfish, this is to keep your network safe because their device could be hacked and your password is remembered on their device.
Don’t operate a wide-open guest network without a password. You don’t know what a neighbor, or a parked car on the street might download on your internet connection under your name. Be safe, put a password on your guest WiFi.