Party City offers an automatic reminder which the host of an event can set to go out between 1 and 10 days prior to the event date. To activate a reminder, head to the Invitation Options page and turn ON the "Send an automatic reminder" option.
You can also designate if you want the reminder to be sent to guests that have responded, guests that haven't responded, or both. If you need to send a "manual" reminder message, just go to your Manage Invitation page and use the "Email Guests" tab.
To set multiple reminders you need to set the earliest reminder first, then go back into the Invitation Options area to set a second reminder.
You will not be able to set a second reminder until the first reminder has been sent out.