According to Zapier documentation :

I want to invite attendees to events via a zap

You can do this by adding a comma delimited list of emails to the Attendees field in the New Detailed Event action.

https://zapier.com/help/common-problems-google-calendar/#i-want-to-invite-attendees-to-events-via-a-zap

But this just doesn’t work because Zapier is using Line Items as follow :

It raise an “invalid attendee email” 400 error

I reached out the amazing Zapier support and here is the trick :

You will need to set up a Utilities Formatter step immediately before the Update Event step, and configure it to use our “Text to Line Item” transformer to turn the comma-separated list of emails into line-items for use on the Attendees field, like so:

Then use your line items in your Zap :