NoMachine (free) allows one connection. If you are working on your local desktop (i.e sitting in front of it) and you would like a colleague or friend to connect to it so that they can collaborate on a document or interact with you, they will be able to connect. This is considered 'one connection'. They must install NoMachine for their operating system on their computer or device. On the other hand, if you are connecting to your desktop remotely, you will not be able to have a friend connect at the same time. This would be considered two connections. If you want two users to connect simultaneously to a remote computer, you should opt for Enterprise Desktop.
To let your friend or colleague connect, 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.