On an instance where sharing is limited to groups, you can see an error when an invitee reacts to a calendar invitation (accept or decline). (Error: “An error occurred while updating the status of your participation. Please contact the administrator directly.”)
As soon as sharing is no longer limited to groups, the invitation confirmation also works with non group member e-mail addresses.
The possibility to invite someone to a calender event should be possible even if sharing ist limited to groups.
See discussion: Error updating event participation by invited attendees in calendar