Reminders and nudges
A timely nudge lifts reply rates more than anything else you can do. kutsu can send them for you, or you can fire one off yourself.
Automatic reminders
On RSVP settings → Automatic reminders there are two switches: a first reminder a couple of weeks before the deadline, and a final reminder a few days before. Both go only to guests who have not answered.
Notify me when someone responds has its own panel below, Notifications for you, and emails you on each reply. It is on every plan, because it writes to you rather than to your guests. Worth turning off for a large event.
To protect inboxes, a guest receives each automatic reminder at most once, no matter how the schedule shifts.
Sending one yourself
Responses has a Send reminder now action for the guests still awaiting reply. For anything more considered, the Emails step lets you write the message, choose the audience and either send immediately or schedule it - see Send invitations by email.
Reminders are email, so they come with Premium. A free event sends no mail at all and these switches stay off.