I came in once and interviewed with three different people (separately). I received a job offer 2-3 days later. The interview was fairly straight forward but the screening process required before you start is lengthy. There is no room for salary negotiations, every individual starting in a given role is offered the same compensation package, regardless of experience, education, etc.
During the initial application process, you'll be required to write a short essay describing why you want to work for ADP. They had a print out of my answers with them in the interview and asked some questions on what I wrote, so make sure you take that portion of the application seriously.