For the past few days I have tried to remember how to solve this, all I can tell you is that you can invite friends if you know the IP number of the Palace. Without anyone else knowing, nobody else should be able to get in. you can use the website:
What Is My IP Address - Shows Your IP Address to find out your public IP.
Will have to install the server to remember how the private palace works but I'm on a mac these days so that might take a while before I get to a windows computer :)
