Short Answer 
To add a new attendee to a meeting in Outlook, open the meeting invite, enter the new attendee’s email in the ‘To’ field without removing existing participants, and then send updates only to those who were added or deleted.
Step 1: Open the Meeting Invite
Begin by locating and opening the specific meeting invite in your Outlook calendar. This is the meeting for which you want to add a new attendee. Ensuring you have the right meeting is crucial to proceed without discrepancies.
Step 2: Add the New Attendee
Next, click on the ‘To’ or ‘Scheduling Assistant’ button to view the current list of attendees. Here, you can directly enter the new attendee’s email address into the ‘To’ field. Be careful not to remove any existing participants; just append the new email.
Step 3: Send Updates Appropriately
Before finalizing your changes, click ‘Send.’ A pop-up will appear with several options. It is essential to choose the ‘Send updates only to added or deleted attendees’ option. This ensures that only the newly added attendee receives the update, preventing unnecessary notifications to other participants.