NoMachine (free) allows one connection. Hence it is for individual use. If you are connecting to your desktop remotely, you will not be able to have a friend connect at the same time. On the other hand, if you are working on your local desktop and you would like a colleague or friend to connect to it, so they can collaborate on a document or interact with you, they will need NoMachine installed on their computer.
You will need to create an account for them on your computer and send them your IP and port number, so they can access over the Internet. To get your IP and port number, open server settings from the monitor or connection GUI. The IP and port details which must be inserted in the connection GUI on the client side will be indicated there. Upon connecting they will see your desktop and be able to interact with you, or just view.