1) Is NoMachine Server a separate product? I installed NoMachine on Ubuntu, and I’m already able to control one machine from the other and vice versa, so does that mean I have both the server and client already?
The NoMachine free product is both a client and a server. So once installed, your computer is ready to be accessed remotely, and you will be able to get access to another NoMachine host.
2) Is there any way I can allow a remote client to see my current session without creating a new user on the server system? I’ve seen instructions for adding a new user to the system. Adding a new user would allow remote clients to see that new user’s screen, and not my screen, right?
An account is required for any user that needs to get access to your local desktop. Installing the free version will allow one connection. So if you are in front of your Ubuntu computer, you can allow another NoMachine user to see what you are working on provided they have an account. If you don’t want them to see your Ubuntu desktop session but prefer them to have a session of their own, what you need to install is Workstation for Linux. This offers the virtual Linux desktop functionality and lets you run 4 virtual Ubuntu desktops simultaneously.