Hi Emma,
To echo Andrew, those people will receive an inviation reminder, since they're not registered. That said, I wanted to share an Event Marketing best practice tip that I've seen work really well! In your Reports tab, you can run
Abandoned Registration report to take a look at anyone who's actually started the registration process, then left. Send that list a more targeted email saying something like, "you're so close!" and encourage them to finish the process. If you use a data tag in the email, it can take them back to where they left off, which your invitees really appreciate!