At my last startup, we grew from 0 to 65 employees without the help of Zenefits and it was painful. Employee on-boarding, HR paperwork, and healthcare enrollment were all done manually.
Now, Zenefits can handle all that for us. We could send new hires the paperwork they need to fill out before they even start. This makes onboarding so much easier.
(Note, I'm not affiliated with Zenefits. Just a big fan.)