I think the easiest thing would be to create a password variable for you game, which someone has to input before the world will let anyone in. Example:

var/global/password=[put password here]
var/global/password_entered=0
mob/Login()
if(global.password_entered)
..()
else
if(input("Please enter the password.","Password")==global.password)
global.password_entered=1
..()
else
alert("Incorrect password.","Denied!")
Logout()

I realise that your world probably has some login code allready in it, and that your password code may need to be slightly modified, but this is only an example.

-Loduwijk