First let’s clarify that NoMachine for Windows doesn’t support terminal server on Windows, so it’s not possible for multiple users to each connect to their own account at the same time. NoMachine free version supports one connection at a time to the remote desktop. Enterprise Desktop, on the other hand, allows unlimited connections and when multiple users connect at the same time, they are sharing the same desktop.
In both cases, if there is no local user (the person sitting in front of the desktop you are connecting to), then you will need to check the box “Require permission to let remote users interact with the desktop” in Security preferences of the server GUI (on the host you want to connect to). This is disabled by default. Checking the box means that users will connect in view only mode. Interaction can be enabled at a later time by the owner.
See the quick guide here:
https://www.nomachine.com/DT11M00106#3.2.