The solution was actually quite obvious now that I think about it. You have to fire up SQL Server in single user mode.
This allows you to log on (using SSMS) from the localhost with sysadmin permissions. From here, you can then create new logins (including domain account logins, if so desired).
Best Answer
If you're doing it via T-SQL:
Granting a Windows Login the ability to connect to SQL Server:
or
If SQL Server-based login:
After that, you can do one of two things. You can add it to the membership of the sysadmin fixed server role using sp_addsrvrolemember:
or you can grant the login CONTROL SERVER permissions: