Override the local Group Policy by domain policy or delete the RSOP

I am working on a new Windows XP computer. It is not connected to the domain. Whenever I try to join a user name to the domain I am facing some issue with updates. I am trying to add the user in admin group. I am doing that through local users and group and then run gpuupdate /force. The policy simply fails to apply. It doesn't work at all. Can anyone guide me on the problem. Thanks.

Get a detailed guide on working with Group Policy. That might really give you information on what you are trying to do. I can tell something about your issue. First I am not able to clearly understand what you are trying to tell. So lets assume that you are trying to change the group membership or something similar to that. It is not possible to add user to the local admin without deleting any existing members.

How to Reset Local Group Policy to Default in Vista, Windows 7, and Windows 8

The Local Group Policy Editor is only available in:
Vista Business, Ultimate, and Enterprise editions.
Windows 7 Professional, Ultimate, and Enterprise editions.
Windows 8 Pro and Enterprise editions.

RD /S /Q "WinDirSystem32GroupPolicyUsers"

RD /S /Q "WinDirSystem32GroupPolicy"

gpupdate /force

Cheers