Readynas Duo v2 - Groups

On the Dashboard, click Configure and go to Security > Groups tab. A fair bit to the right of that you should see a refresh button and just to the right of that you should see a button with a + on it. Click on that and you'll get a dialog box to add a group. Then you can add secondary users to the group (if you want them to remain in the group they're already in) or go back to the users tab and change the primary group to which a user belongs.