RS485 port
The USB port on the inverter can not be used for monitoring. You need to connect it via an RS485 cable by screwing the RS485A and RS485B wire into pin 3 and 4 of the communication port. Each inverter will require it's own USB cable.
You can also explore alternative RS485 adapters that might be available locally in your country.
Steps in SolarAssistant
On the configuration page, select "Growatt" as your inverter model.

Devices connected via USB will show up. Select all ports that are Growatt inverters.

Click connect:

Parallel installations
With Growatt inverters you will need to connect each inverter with its own cable.