How to add a group to the sysadmin fixed server role in SQL Server 2008 R2?
As stated, SQL 2008 R2 no longer adds the administrators group as a SQL login by default. You first have to add the Administrators group as a SQL Server principal.
CREATE LOGIN [BUILTIN\Administrators] FROM WINDOWS
(or see this article for more information on how to add a login)
You can then add that group to the sysadmin role.
sp_addsrvrolemember
@LogiName='BUILTIN\Administrators',
@RoleName='sysadmin'