To add optional attendees to an Outlook event, you can follow these steps:

1. Choose the Calendar icon in the lower-left corner and click the “New Meeting” option under the Home ribbon.
2. Enter the email address of the primary attendees in the “To” box.
3. Select the “+Optional” button next to the “To” box to expand the window.
4. Enter the email address of the optional attendees in the “To” box.
5. Alternatively, you can click the Scheduling Assistant tab and click the plus button ( + ) to add a new invitee and search for someone in your Contacts list.

You can also add an optional message to the attendees. Once you are done, click the “Send” button to schedule the meeting and add the optional attendees.

I hope this helps!