OG: Only allow certain Drupal roles to join a group
I don't think that functionality is packaged with OG on its own.  

Are you looking to map certain Drupal roles to certain groups or only have the ability to apply to a group available to certain roles? 

I would think this hook is a place to start:
http://www.drupalcontrib.org/api/drupal/contributions%21og%21og.api.php/function/hook_og_permission/7

But here is a list of all hooks you can use with OG:
http://www.drupalcontrib.org/api/drupal/contributions!og!og.api.php/7