How to add Microsoft Forms registrants to an existing Outlook calendar event with Power Automate

Create the event in your Outlook calendar first (the demo is a one-hour session called Power Automate Automating Business Processes). Every registrant will be added to this one event. It can already have attendees. The flow keeps them.

Open Microsoft Graph Explorer and sign in with the account that owns the event. On the Modify permissions tab, consent to the calendar permissions so it can read your events.

Run a GET request with this URI:

https://graph.microsoft.com/v1.0/me/calendar/events?$select=id,subject,start,end

Find your event in the response, copy its id and paste it into Notepad. You need it twice in the flow.

Put the event date, time and location in the form description so people know what they are registering for. The form has three questions: Name, Email Address and a privacy consent.

In Power Automate select Create > Automated cloud flow, name it (the demo uses Event Registration to Outlook Calendar) and choose the Microsoft Forms trigger When a new response is submitted. Pick your form.

Add Get response details, select the same form and set Response Id to the trigger's Response Id. You now have the registrant's name and email.

Add Send an HTTP request from the Office 365 Outlook connector (a standard action) and rename it HTTP Get Event. Use method GET and this URI, with your event ID in place of YOUR_EVENT_ID:

https://graph.microsoft.com/v1.0/me/events/YOUR_EVENT_ID?$select=attendees

The $select=attendees part returns only the attendee list. You need it because the update must send back the complete list: existing attendees plus the new person.

Add Parse JSON. Set Content to the body output of HTTP Get Event. For Schema, select Use sample payload to generate schema, paste a small sample of the attendees response and select Done. Power Automate generates the schema so you can pick attendee fields as dynamic content.

Add Initialize variable, rename it AllAttendees, set Name to AllAttendees and Type to Array. Leave it empty. It will hold the final attendee list.

Add Apply to each. For the input use an expression that returns only the attendees from Parse JSON, for example body('Parse_JSON')?['attendees'].

Inside the loop add Append to array variable, rename it Add existing attendee, choose AllAttendees and paste an attendee object as the value, for example:

{"emailAddress":{"address":"","name":""},"type":"required"}

Put the cursor between the quotes of address and insert the attendee's emailAddress address from Parse JSON (under See more), then do the same for name.

Outside the loop, add another Append to array variable, rename it Add new registrant and choose AllAttendees. Paste the same JSON structure, but this time insert the Email Address and Name answers from Get response details.

Add another Office 365 Outlook Send an HTTP request and rename it HTTP Update Event. Use this URI with your event ID:

https://graph.microsoft.com/v1.0/me/events/YOUR_EVENT_ID

Set Method to PATCH (it updates only part of the event). Under Advanced parameters add Body and send the full array, for example {"attendees": @{variables('AllAttendees')}}. Save the flow.

Select Test > Manually and submit the form with a test name and email. After the run succeeds, reopen the event in Outlook and select Edit: the new attendee is listed. The registrant's inbox has the invite, and your Sent Items shows only one email, to the new person.