Do I actually need an HR function if I have five people?
You need the functions, not the title. Someone has to draft offer letters, track PTO, onboard new hires, and keep compliance current — if you do not assign it, it falls to the CEO, which is the worst possible outcome. The AI HR Manager runs these functions at the quality of a competent fractional HR lead without the $3K/month retainer. Once you cross 20 people you will likely want a human HR lead; below that, the AI HR Manager plus Gusto is genuinely enough.