You have to create your users first, then enable access restrictions. When you enable access restrictions you have to highlight the users then use the arrows to kick them to the read or read/write categories.
When a use maps a drive to the nas it will ask for login information (to map go to my computer/tools/map network drive). Then you can access the directory your looking for.
Multiple users can not log into the admin interface (GUI) with out getting that error message.
TIP: Map via IP address not name (Example \\192.168.11.150\share )