Adding users to a FBA site

I'm using SharePoint Foundation 2013. I have created an Extranet where Active Directory and Form based users could log in (multi mode). I need to add users to this site via a windows service. My question is of two parts.

1. 
   

   By default there is a Reader group created when the site is created. How can I add a user to this default group? (Rather than hard coding group name)

   
   


2. 
   

   Is there a recommended approach to add Form based users to the site? (I am storing the form based users in a SQL DB table.)