Create login script to make users local administrator

I prefer Jay_Jay70's suggestion of Restricted Groups, but below is a little batch file I have used. Good option if you quickly want to add a few users from time to time.:

Copy from the Windows 2000 Resource Kit or from http://www.activexperts.com/activmonitor/windowsmanagement/reskit2000/ 
the file cusrmgr.exe and put it in a folder of your choice. I recommend doing this from the domain controller but works from any workstation in the domain so long as you are logged in as a domain admin.
In the same folder create a batch file named Add.bat and insert the text below;
==========================================================================

:: Batch file to add username %1 to local Administrators group on Computer %2
Echo off
CLS
If Exist UserAdd.log GoTo START
Echo Results from Add.bat > UserAdd.log
Echo Note: "Failure" usually indicates user/group etc. already exists in local group >> UserAdd.log
Echo       "Can not get SID" usually indicates Computer name is wrong or unavailable >> UserAdd.log
:START
Echo. >>UserAdd.log
Echo Add %1 to %2 >>UserAdd.log
cusrmgr.exe -m \\%2 -alg "Administrators" -u %1 >> UserAdd.log

==========================================================================
Now simply run by going to a command prompt. Change to the directory where you put your files and enter:
   Add username computername
You can substitute groupname for username. If there is a space such as Domain Users enclose in quotes: "Domain Users"
I thought the username had to be in username@domain.local but the basic name seems to work fine, if you have problems use the long form. No "\\" are necessary for the computername.
It will also create a log file named UserAdd.log where you can check for success or errors.

I don't like Restricted groups because "When you configure the members of a group, it will overwrite the existing membership of the group and replace the members with those specified within the GPO.". This limits the usefulness of RG for me.

See https://www.experts-exchange.com/questions/21134622/Add-Active-Directory-group-to-local-Win2K-Group.html for SaintBA's VBS solution or use see my amendment of RobWill's batch file. Change set @User=Domain\Username to match your requirements.

:: Batch file to add specific username to local Administrators group for each computer in workstations.txt
Echo off
setlocal
CLS
set @User=Domain\Username

If Exist UserAdd.log GoTo START
Echo Results from Add.bat > UserAdd.log
Echo Note: "Failure" usually indicates user/group etc. already exists in local group >> UserAdd.log
Echo       "Can not get SID" usually indicates Computer name is wrong or unavailable >> UserAdd.log
:START
Echo. >>UserAdd.log
for /f %%i in (workstations.txt) do call :parse %%i

goto end

:parse
Echo Add %@User% to %2 >>UserAdd.log
cusrmgr.exe -m \\%1 -alg "Administrators" -u "%@User%" >> UserAdd.log

:end

Restricted groups has 2 options "members of this group" (normal) and "This group is a member of". The latter will allow you to add members to a local administrators group, where the former as suggested, will replace all existing members except the Administrator account.

Note: Be careful using restricted groups that you don't apply it to your domain controller, or if you do so be very careful as it is possible to lock yourself out, even as a domain administrator.

Some useful Restricted groups links:
http://www.microsoft.com/resources/documentation/windows/xp/all/proddocs/en-us/sag_scerestrictgroups.mspx   <READ CAUTION SEGMENT>
http://www.microsoft.com/resources/documentation/windows/xp/all/proddocs/en-us/sce_res_group.mspx
http://www.windowsecurity.com/articles/Using-Restricted-Groups.html
http://www.msresource.net/content/view/45/47