Hi,
What is your WSS or SharePoint server’s version?
Did you install Windows SharePoint Services 3.0 February 24, 2009 cumulative update? If not, try to install this update and check the effect.
Issues that this hotfix package fixes:
· When you try to modify group membership of the built-in groups of site collections, you may receive a "System.UnauthorizedAccessException" exception. For example, this issue may occur when you try to add a new user to a built-in group. Built-in groups include visitors, owners, members, and so on.
Description of the Windows SharePoint Services 3.0 hotfix package (Sts.msp): February 24, 2009
http://support.microsoft.com/kb/961750/
Hope this helps.
Rock Wang
Rock Wang– MSFT