Hi there @pierreb,
Welcome to the Community! 
I did some digging into this, and it seems like in order to add invitees to all instances of the recurring meeting in Google Calendar, you need to make a small change to your existing Zap.
Please follow these steps:
- Add a "Find Event" action to find the next instance of the recurring event
- Locate the “Add Attendee/s to Event” action in your current Zap.
- Map the Recurring Event ID in the “Event” field as a custom value that you retrieved from the “Find Event” step. Here’s an example screenshot:

(view larger)
By making this change, the Zap will add the new attendee to the entire series of recurring events using the Recurring Event ID instead of just a single instance.
Hopefully, this helps! 