First I've heard of it (but I have been busy!) but easy to fix:
Open Safari and log in to iCloud.com where you open the Calendar app. In the bottom left corner, click the cog and choose Preferences. Click the Advanced icon and elect to have invitations sent by email rather than in app. When an unknown person sends an invitation, just delete the email - they will not be notified it's been received.
I doubt you're the only member of the group to have received them, just the only member of the group who reads the discussions to have received them ;-)