"calling_ply" is a variable ULX uses to store the player that is calling the command. When in the function of the command, you can call this, like "calling_ply:Nick()" to display the name of the person who called the command. If you're passing the argument, I think you can edit who is considered the calling_ply if you pass it. Not sure, though.
Also, net.Receive takes two arguments to its function, Len and ply. The first is Len (length of the message) and ply the second. Even if you're not using Len, you need to specify it (or just put a placeholder)