Our tool is a Node.js server / React client running on a network with no external web access.
We would like to add robust user management including:
Is there any open source libraries out there that can supply these capabilities?
Passport js library provides a lot of strategies for authentication and authorization without requiring you to reinvent the wheel. You can use any of the databases to store the user's on the backend like MongoDB, Mysql, etc.
It can also talk to inhouse LDAP or auth server API if you decide to do so. All in all, this sounds like a perfect library for you.
Here is a tutorial https://scotch.io/tutorials/easy-node-authentication-setup-and-local