I recently was setting up a new Microsoft SharePoint 2010 machine and had promoted the machine to a domain controller before creating my SharePoint admin accounts. I needed to add several of my accounts to the local Administrators group. Unfortunately after you promote a server to a domain controller you can no longer access the GUI for Local Users and Groups. Instead I had to use the command line to add the users.
Open a command promt using the “Run as administrator” function and then run the following command.
net localgroup Administrators /add {domain}{user}
eg: C:> net localgroup Administrators /delete MYDOMAINusername