You don’t need SSO for users to have an account they can use to log in.
The computer needs to be on for as long as you want AMP and the MC server to be running (which in some cases is 24/7).
You’d select Standalone, since you’re not setting up a mult-machine deployment.
In terms of getting the modded MC server set up:
https://discourse.cubecoders.com/t/setting-up-a-modded-java-minecraft-server/26535
And for setting up user permissions see:
https://discourse.cubecoders.com/t/managing-user-permissions-in-amp/2301
(If you meant them joining the game server, that would be unrelated to users in-AMP)
And finally, if you want the web interface to be accessible externally, consider setting up HTTPS to make the web page secure:
https://discourse.cubecoders.com/t/setting-up-secure-http-https-with-amp/2305#h-2-reverse-proxy-on-windows-5