It is trivial for other people to log into your MUD and play it because the client software is telnet and everyone has access to telnet. However, things get more challenging with the graphical game because you wrote the client and they have to run the program that you created. This document describes the steps you need to take to let other people run your graphic game.
Make a fat JAR - I included sbt-assembly in the initial setup so that you can go into sbt and run the "assembly" command. This will create a fat JAR file. This is a Java Archive file with all the dependencies that people need to run your program.
After you run assembly, copy the file it creates into your home directory.
Use chmod to give everyone read permissions on your assembly file. In your home directory you would type "chmod a+r assembly.jar" where you put the proper name of your assembly file.
Run your server.
On the command line have your friend run the client. They can do this with a command like this: scala -cp /users/username/assembly.jar graphicgame.ClientMain. Replace any parts of that with details that fit you and your program.
Note that for this to work well, you need to let the user provide a machine name for the server. This could be done using readLine when the program first starts or using scalafx.scene.control.TextInputDialog.