To assign multiple employees to multiple payroll groups simultaneously, go to Employees and click Mass Update Employees.  

A prompt box will pop up. Click Employee 201 Upload Field Guide.  

You will be directed to the Field Guide, which gives instructions on how to fill out the Mass Update Employees template.  Click Download Sample CSV file.

Pull up the CSV and delete all of the columns except Employee ID and Payroll Group.  Enter the employee IDs of the employees you want to add to the payroll group, as well as the payroll group name of the payroll group you want to assign them to.  This also applies if you have multiple payroll groups.  Save your CSV.

Go back to Employees and click Mass Update Employees.  The prompt box will pop up.  Click Choose File and select the CSV you just saved.  Click Upload.

Some errors may occur if you fail to provide the right information. Correct the CSV in accordance with the message and upload again.