Hi GraemeW77!
Thank you for reaching out to the Community with your question.
When the Add to my calendar link for an Event, or Add to calendar link for an RSVP is clicked, your registrants and contacts have the option to save or download an ICS file (part of iCalendar) that can be used to add the event to their default personal calendar, including a Google Calendar.
Depending on the type of calendar and settings your registrants have on their computers, clicking the link might automatically open an appointment for them, or they might have to click on the ICS file manually to create the appointment on their calendar, or they may even need to import the file into their calendar.
For information on importing an ICS file into a Google Calendar, please see Google's support and for information on importing an ICS file into Outlook, please see Microsoft's support.