OG: Only allow certain Drupal roles to join a group I think this might be a very simple setting I am missing somewhere, but I want to prevent people from joining an 'Organic Groups' group unless they have a particular Drupal role. The 'subscribe to group' permission in the group obviously only applies to OG roles, not Drupal's core roles.

I can't see any permission in Drupal's core for 'join group type', which I would have expected.

Any ideas?
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