How do I add employee si number when employee quits and comes back in a month?

Hi usercraig_sue,

Welcome to Community!  It's important that you're able to add your employee to QuickBooks Online Payroll and ensure they're paid as scheduled.  I'll be happy to assist!

Based on the information you've provided, it's not necessary to add the employee to your books upon their return.  In this situation, you'll just want to make them active again and make any necessary changes to their employee profile and pay schedule as needed. 

Here's how to make the employee active; 

1. Open Payroll from the left menu and then Employees 

2. Above your employee list, select the ▼ drop-down for Active employees and switch it to Inactive employees

3. Click on the employee's Name, then click the Edit ✎ next to Employment

4. Update their employment Status as needed then click Done.

Please don't hesitate to reach back out if you have any other questions.  We'd be glad to help!