OK. A build with user authentication is now up on the download server (and dockerhub for docker users). The branch for this test is called userauth (as opposed to latest).
Bare-metal users will have the easiest time installing, because you just need to download the userauth package instead of the latest package, and then install/update as usual.
Docker users will likely find it harder to make this switch. If you are using docker-compose, you can just change the image name in your config file from latest to userauth; then update (docker-compose pull) and start. But for NAS users and those not using docker-compose, you will need to rebuild your container. You can keep your configs and storage data; just make sure your new container points to the same directory where all that's stored (everything in the container configuration should be the same). See the installation instructions for reference, and look at/capture the settings of your current container before you launch into rebuilding it with the new image.
To configure access control, see the documentation.
If you run into any showstoppers, just rename your users.yaml file to something else (e.g. Xusers.yaml or users.yaml-disabled and restart Reactor, and that will disable access control. Otherwise, this release is identical to 24115, and you can switch between the two without problems.