Charge HQ can be used by anyone who has compatible equipment. It is available in all countries. It is currently in a beta phase, which means frequent changes and occasional bugs. During the beta phase, Charge HQ is free to use.
To get started, download the mobile app and follow the instructions in the app.
You can also access Charge HQ via a web browser at: https://app.chargehq.net.
The web app is the same as the mobile app except it can't do the initial connection to a Tesla account (you need to use the mobile app for that) but it can do everything else and is often the easiest way to setup any account where you need to copy-and-paste keys, e.g. solar inverter credentials. Both the web app and the mobile app can be used in parallel.
Once you have the app, getting set up usually only takes a few minutes, although it may take a bit longer if you need to configure a smart charger. The following articles may be helpful when getting started:
You'll find many more guides and FAQs in the support section.
There is also a Facebook group for members of the beta program. It can be used for asking questions, raising issues, making suggestions or any topic related to Charge HQ and EV charging.