The other company I've worked at that chose not to use Active Directory was Tenable. They even acquired a company that specializes in AD security, but ended up deciding AD was too insecure for their IT infrastructure.
https://www.rippling.com/products/hr/peo
If you work at a small company, HR, benefits, tax reporting is handled by Rippling as if you are their employee even though for everything else is handled by your employer. You are “co-employed” by both companies. As our company grew, last year we all got “terminated” by Rippling (initiated by our company) and had to “re-apply” to our company as far as new tax forms.
Nothing changed day to day. It was merely an accounting, legal, tax thing.