I have also used Mint 13 on a number of occasions for single-task systems that just require a single user to do a single job. But today I went to add a second user on a small system I am testing and I found that the basic installation of Mint 13 XFCE does not include the graphical tools to add new users. You'd expect to find these by clicking on Menu then selecting System and then Users and Groups.
Here is a simple one-line command to add this tool back in:
Open Terminal and enter the following line:
$ sudo apt-get install gnome-system-tools
Once the command has finished executing you can find the graphical tools to add users under System --> Users and Groups