You have to set up your hotspot with a network name and password, just like any other wifi network. There is a default network name and password you can view or change, then people can sign in. Settings/connections/mobile hotspot and tethering. Tap on that and then tap on mobile hotspot. You will see the current network name and password. You can keep them or change them.
The easy way to get to that setting is to long press on the mobile hotspot setting in the quick panel. It will bring you right to the network credentials.
