I have over 22 years in accounting as a full charge bookkeeper which include doing HR and payroll, basically if you understand the Federal payroll laws concerning overtime, hiring etc. and read and learn to understand all laws concerning hiring, terminations, garnishments, benefits, payroll taxes, you have to be currently with all the changes occurring within the last years.
usually I paid an average of about 20 employees per company. Until the last company, I started in Accounts Payable Specialist and was promoted to Payroll Supervisor with about 500 employees, this was also In-house payroll.
The object is know your State, Federal laws pertaining to all payroll, how to handle new hires and terminations, benefits etc. As long as you are willing to learn and read a lot concerning payroll. Take a few courses - basically the state will hold payroll work shops for all HR, payroll employees to keep current with new laws.