You can download it via Epic Game Store without joining the org. Github organization is for contribution from non-employees as well as source code for various projects (like engine demos).
Only for those still using Windows. The head of Epic has some bizarre complex against Linux, so they refuse to release binaries (or the Epic Game Store at all) for Linux. So the only way to install Unreal Engine is to link your Github account to their org, clone it, then build it from source. Which takes a few hours and something like 70gb of disk space while building.
