How to add deleted user in sql server?
You can try below alternatives:


If you installed SQL Server then you will be having "sa" - if mixed mode authentication is enabled. You can use "sa" account to create a login and map that login to a database.
Alternatively, you can use "DAC" (if you have enabled it as a part of post installation step).
By default, BUILTIN\Administrators has access to sql server 2008 instance. If you are part of BUILTIN\Administrators group then you will have access.
Last resort : Recover access to a SQL Server instance by Aaron Bertrand wherein he shows how to use PsExec to get into sql server.


Once you gain access to sql server, then create a login and map that login to a database. 

Make sure to NOT delete it again :-)