Offering health coverage only to select employees

Q. We currently provide health insurance for 20 employees who work full time (40 hours per week all year). We have some clerical staff who work less than 40 hours per week and aren’t eligible for insurance. We also have professional tax preparers who work many hours during tax season, but only 10 to 20 hours the rest of the year. Can we legally add just the tax preparers to our health insurance and not the clerical staff? —R.M., Pennsylvania