Allow me to share some knowledge regarding QuickBooks Online Standard Payroll (QBOP), and how it works.
Once you've selected a pay schedule and set a pay date, you can still change it as long as you haven't run your first payroll yet.
You can go back and edit via these steps:
Go to the Gear icon, then select Account and Settings.
Move to the Payroll tab.
Select the Pay schedule section and make your changes.
Click Save pay schedule.
Going back to your question, if you've added an employee after your current pay date then that specific employee won't be included when you run payroll, but will be in the next one.
If you already ran your first payroll, your payroll settings will be locked until you delete the pay run.