How do I remind guests who haven't answered?
Three days after your invitations go out, your Overview offers Nudge who hasn't answered, a reminder already written for them. To keep reminding them on a schedule, send a repeating email that stops for each guest once they answer.
The nudge
Three days after the last invitations went out, while somebody still has no answer, the top of your event's Overview says how many invitations have no answer yet, with Nudge who hasn't answered beside it. Press it and Messaging opens with a reminder already written and addressed to Not yet responded: only the guests who haven't answered. Read it over, change what you like, and send it. Once a nudge has gone or is scheduled, the Overview waits three days before offering another.
The bell in the header can say the same, when no answers have come in or answers are slow. If you haven't opened the event for a couple of days, we may email you about it too; turn that off with Reminders under Emails to you in your account.
Remind them on a schedule
To keep reminding guests without coming back to do it, press New message in Manage → Messaging, write the reminder, and choose Not yet responded under To. On Send, choose Repeating email: when the first one goes, how many days apart the rest are, and when it stops. Choose They answer and each guest drops out the moment they reply, so nobody who has answered hears from it again. You can also stop it at The event, or After a few emails each. A repeating email goes by email only, never as a text.
See How do I manage scheduled and automatic emails? for stopping or changing one, and How do I send guests an update? for the composer.
Guests who said yes
The reminder before the party is a different email: the Event reminder, in Messaging's Settings, goes to guests who said yes, at the time before the event you choose.
Updated