Why: I have homelab environment where I use freeIPA as my centralized base fora authentication and DNS server in this environment and it will be very nice If I can use it to login into Solus machines. I think that this is much more than common LDAP and I like the automation its gave me when I adding new machine or new user to my environment installing freeipa-client.
Is it Open Source: I think yes (https://www.freeipa.org/page/About)
How many users do you anticipate will use this software? I don't know.
Link to source tarball/zip file: https://releases.pagure.org/freeipa/freeipa-4.5.0.tar.gz I think that client alone is in this package (https://github.com/freeipa/freeipa/tree/master/client)