You press F10 which brings up the GUI visual editor. You need to have downloaded the Mission Editor to have access to F10. To make them open a GUI by typing /help they would have to have some client-side code and then the server command would call the client command. You need a client command because otherwise if you just put canvas.pushDialog(HelpGUI); into the server command then whenever somebody typed /help then the Host would open the GUI.
Alternatively you could use the built in OK box to display your message, then the client doesn't have to download anything as they already have the client command. I think the command's commandtoclient(%client,'MessageBoxOk',"Title","Message"); I'm not 100% sure though.