The "SSInvitationAgent" is actually a component of other applications such as Apple Messages that also offers a screen sharing option, which is separate from the Mac-System Preferences-Sharing indicated in a warning when starting LockDown Browser for Mac.

If you see the prompt about "SSInvitationAgent":

1. Close LockDown Browser.

2. Verify you are signed out and have closed Apple Messages (iMessage) and other messaging apps.
 
3. Go to Finder-Applications-Utilities and start the "Activity Monitor".

4. Click on the "CPU" tab, click on the "Process Name" column to sort the list

5. Locate both "SSInvitationAgent" and "SSMenuAgent".

6. Highlight each process and click the "Stop" button as seen in this screen capture:

You may need to select "Force Quit" a couple of times before the process will finally stop.

7. Restart LockDown Browser for Mac.