When I ran SQLEXPRADV_x86_ENU.exe (run under Windows' Administrator user account) to install SQL Server 2008 Express with Advanced Services, when it got to the Database Engine Configuration step, under "Specify SQL Server administrators" I clicked on "Add Current User" to assign NULL\Administrator as a SQL Server administrator.
How can I now add my other Windows user accounts (both administrator level and reduced-privilege accounts, if possible) as SQL Server administrators?