How can I assign my existing workforce to a labor group in Payroll?

You need to set up specific labor groups and their corresponding codes in the Company Profile. Once an employee is assigned to these labor fields, they will be available in Payroll for you to manage during the Edit Pay Run step. Please keep in mind that the default Location and Department of an employee would be available in the Labor Distribution Report and Labor Distribution Detail Reports, if you don't add any labor groups during the Edit Pay Run step.