A Unity ID allows you to buy and/or subscribe to Unity products and services, shop in the Asset Store and participate in the Unity community.
Separate names with a comma.
I had the installer hang on Windows as well and found that an error dialog (visual studio encountered an error) was waiting in the background....
Looking at it now at least they made an effort to point it out by highlighting the text, though I didn't get it either - interesting (aka "poor")...