A sys admin is maintaining an application on AWS. The application is installed on EC2 and user has configured ELB and Auto Scaling. Considering future load incr...

A sys admin is maintaining an application on AWS. The application is installed on EC2 and user has configured ELB and Auto Scaling. Considering future load increase, the user is planning to launch new servers proactively so that they get registered with ELB.
How can the user add these instances with Auto Scaling?