You can send out an E-Bill reminder at any time to your members with a balance. An email will get sent to the member's email on file reminding them to make a payment. You can send one yourself to test and review what the email says.
Invoice tab > E-Bill Reminder > Manage
Select the member with a balance and click Select Reminder
NOTE: If you click the square in the upper left-hand corner it will allow you to Select all.