Hi @elio 

To know who's using your WiFi connection you can follow 2 methods: 

Method 1: Using apps and softwares

There are a lot of apps and softwares available which can show you how many and which devices are connected to your wifi network.

For android phones:

Who is on my WiFi is the best app which will help you detect and block unauthorized users from your wifi.

It is only 5.5MB and has over 1 million downloads.

For iOS users:

Fing – Network Scanner an iOS app that allows you to see all the connected devices, their operating systems and the nature of their connection, MAC address and many more parameters.

For Desktop users:

1.) Wireless Network Watcher is one of the best network analyzer software that requires no setting up of IP address ranges and starts scanning your main network adapter automatically for connected devices.

Once it scans the IP addresses on your WiFi network (which it does very fast) you will get information like IP address, device name, MAC address, device manufacturer, and detection count.

To know Method 2 visit: How to Check If Someone is Using My WiFi 

How to know how much bandwidth they are using? 

Visit How to Give Bandwidth Priority to an Application over Another for more details.