Fedora 16 User Manager: can't edit Group Properties

[John Pilkington]

Hi: My f16 installation has been successively upgraded from about f12. 
My tv cards are recognised by the kernel, but ioctl problems have 
interfered with the setting up of new cards while not affecting the use 
of those already known.   A post I saw today claimed that apparently 
similar problems in Ubuntu could be cured by re-adding a user somehow 
lost from the video group.

http://docs.fedoraproject.org/en-US/Fedora/16/html/System_Administrators_Guide/s2-redhat-config-users-group-properties.html

My attempts to add a user with the User Manager fail.  I'm told the 
video group already exists, try a new name.  I deleted and re-created 
the (empty) video group and still could not add any users to it.  Now 
the two User Manager windows cannot be closed; attempts to minimise them 
just redraw them.   Any suggestions gratefully received.

John P