When Outlook receives a meeting acceptance, the sender of the acceptance is compared to the recipients in the meeting attendee list. If that recipient is not listed, it is assumed to be 'optional' and added to the list of attendees. This is by design that I am aware of. When the name is displayed in the search results, you have additional options to select Required or Optional for the meeting invitation. By default, when you click OK, the person is added as Required for the meeting invitation, and is listed under the All Attendees area in the Scheduling Assistant view.

Invite Optional Attendees Outlook

When sending a meeting invitation you cannot add someone as a CC or a BCC to the meeting invite. Use the following instructions that detail how to define optional attendees in the meeting invitation.

How to modify groups using Outlook Web Access (OWA)

Invite Optional Attendees In Outlook Web

1. Go to your Outlook calendar by clicking on the squares icon in the top-left corner and select Calendar.
2. In the Outlook calendar click New then select Calendar Event.
3. In the new meeting window that opens, select Scheduling Assistant.
4. In the Add Attendees field type in the name of an optional person you wish to add to the meeting.
5. Right click on the name of the attendee and select Attendance optional.

How to modify groups using the Outlook application

1. In your Outlook calendar, click on the New Meeting icon in the ribbon.
2. In the new meeting windows that opens, Click on Scheduling Assistant.
3. Click on the Add Attendees button at the bottom left of the Attendees list.
4. In the Select Attendees and Resources window, you can add people as required or optional. Select the person you wish to add click on either the Required or the Optional button to add them to the invite.