[xubuntu] real noob user/groups question

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http://ubuntuforums.org – hi, I am up the only user on my system ( up until a week ago ), and therefore I am the admin with sudo rights. I have my own home folder. So recently I decided to create another user account for someone, and now I have just discovered that they have been wandering around anywhere they want to, into my home folder,the system, etc., you name it, and look at all the files and read them. I don't know much about adding users on my system, but I don't think it's supposed to work that way. The user is not a member of any group except their own. I thought a new user was supposed to be confine (Hardware)