When querying on event registrants you cannot pull anyone into the query who did not register for the event. Anyone who was sent an event invitation but did not reply will NOT pull into the query.
We are currently evaluating this issue and will update this article when we have more information.
Add an additional qualifier in the query that asks for everyone that the invitation was sent to. This will include everyone that is not captured by the term "registrant". Example: Communications\Event Invitations\Name is equal to ______________________________.