Hey Husse, it has taken me months to figure this out and the OP might need this knowledge too.
The 'Users' part of 'Users and Groups' shows next to nothing by default - not even your own user account
If you want it to be the slightest bit useful you have to run gconf-editor and go to 'apps/gnome-system-tools/users' and place a tick in the 'showall' box.
Why on earth is it this obscure??
Fujitsu Lifebook AH532. Intel i5 processor, 6Gb ram, Intel HD3000 graphics, Intel Audio/wifi. Realtek RTL8111/8168B Ethernet.Lubuntu 13.10,Ubuntu12.10 (Unity), Mint16 (Cinnamon), Manjaro (Xfce).