Former Outlook MVP Jay Harlow provided this information in a newsgroup posting:
In Microsoft Outlook, making someone an 'Optional' attendee is the same as Cc'ing that person. Making someone an 'Resource' attendee is the same as Bcc'ing that person.
To CC (or BCC) someone to a meeting
To CC someone to a meeting, you can use one of three methods to place the address in the Optional attendee field. (To BCC, use the Resources field and see the Notes, below.)
1. Select the Attendee Availability tab (Outlook 98/2000), Scheduling tab (Outlook 2002/2003), or Scheduling Assistant (Outlook 2007/2010)
Click in the Attendance column, and change the attendance for the person to Optional Attendee. Choose Resource if you want the person BCC'd.
2. On the Scheduling Assistant page, click the Add Attendees button to open the address book dialog and select invitees.
3. You can also click the To button on the Appointment page to open the address book dialog and put the person in the Optional (or Resource) list.
Forward as an iCal
Another method for BCC'ing invitees is to forward the meeting as an iCal. Add the attendees to the message BCC field. When the invitees accept or decline the invitation, the organizer will receive the response.
The Sender will see the Resource names in the To field of the meeting request, but when they click the To button, the addresses added as Resources will be in the Resources field.
Resources will be listed in the Location field - the sender will need to delete the BCC'd names from the Location field. If the Location field is empty, type in a location, otherwise Outlook will add the Resources back to the Location field.
This screen shot shows what the recipients see. If someone is CC'd, their name or address would be in the Optional field.
There are many reasons why when sending invitations to the meeting we do not want each of the invited guests to know the e-mail addresses of the other invitees. Outlook users can help themselves by using Outlook Tiny DLP, which solves this and many other problems related to information security and GDPR requirements. The basic version of this add-on, solving problems related to the use of the BCC field in e-mails and invitations, is available free of charge. Tested with Windows 7+ and Outlook 13+.