Short Answer 
To add a new attendee to a meeting in Outlook, open the meeting invite from your calendar, enter the new attendee’s email in the ‘To’ field, and then send the update only to the new attendee by selecting the appropriate option before sending.
Step 1: Open the Meeting Invite
To begin, navigate to your Outlook calendar and locate the relevant meeting invite that you wish to modify. Double-click on the event to open it. This is crucial as it allows you to access the list of current attendees and make the necessary changes.
Step 2: Add the New Attendee
Once the meeting invite is open, find the ‘To’ or ‘Scheduling Assist’ button. Click on it to display the list of current attendees. Here, you will enter the new attendee’s email address directly into the ‘To’ field, ensuring that the existing attendees remain unchanged.
Step 3: Send the Update Selectively
After adding the new attendee’s email, click on the ‘Send’ button. A pop-up will appear, allowing you to choose specific sending options. Make sure to select ‘Send updates only to added or deleted attendees’. This way, only the newly added attendee will receive a notification, while current participants will not be disturbed.