HA is not really well suited to handle a billing system. There is no obvious way to support that within the sensor model of HA which only presents real-time data. The Easee API does also not provide a way to see which user is charging at a given time, this data can only be retrieved after a charging session is completed.

It is a better option to use the builtin function in the easee cloud where lists can be downloaded or create a python script that suits your needs. There is an example script in the pyeasee project that can be used as a starting point: https://github.com/nordicopen/pyeasee/blob/master/examples/user_consumption.py