You designate an entrance/door (and might I suggest a secret entrance if possible?), and you would decide on a password such as "banana split". Then you make sure you can open the door but no one else from the inside. When someone comes to enter, you ask for the password. If they don't know it, they can't come in. And you only tell those you want in. And you can change the password in case you don't want current people to come in at a later time.