Ask HN: Best SaaS to offload all the user management part out of the product?

We are looking for a Saas product that could offload all the user management part out of the core product, i.e. a tool that handle loads of tedious things related to user management, like: sign up, authentication, password recovery, billing status and subscription, front-facing self-serve for user account and download invoices page, etc. Any recommandations ?