How to know who connect to my wifi

To know who connect to your wifi network is using the ip (internet protocol) scan software.

For smartphone there are many ip scan App, you can use it to know who connect to your wifi network.
The App name I recommend for you is Fing, support for Android or iOS.
Go to Play store (Android) or App Store (iOS) then search the App using keyword: fing
Select Fing-Network tool by Domotz.Ltd, and install it.

Connect your smartphone to the wifi that you want to see who are connecting to.
Run the Fing App
You will see the result, tap refresh icon to refresh.

below is the example, captured from my smartphone:

The name of devices, ip address and MAC address will be shown.
MAC address can be use to set filtering in wifi/router to pass or denied the connection.