Single member LLC or LLC

Quick question - are you paying them as contractors or as full employees with tax deductions, etc.? As for chaning from a Single Member to a regular LLC, most states allow you to change items like this through your Secretary of State's office - many times online. That's a good first place to check. If you are paying them as employees, you might look at a company such as Paychex who can manage all of that for you and cut tax documents rather than doing it yourself. You'll avoid errors, save time and it's pretty reasonable for costs.

As long as your employees are really employees - that is hired help and are not going to have an ownership stake in the company (nor contribute equity into the business) there is no need to do anything the the legal structure. The only time to update the structure is if you bring on additional owners.
As for the payroll question, by legal definition an employee is someone whom the company controls their time and responsibilities versus contract labor that is free to come and go as they please. If the people are employees you should have them complete the W4 and I9 forms for their withholding allowance. Then calculate the amount of withholding you should have taken out by using an accountant or Circular E, fill out a federal form 940 and 941 declaring the total payroll for the year and calculate what you should have withheld and the employer's share, add 10% for penalty.