How to let an OG administrator contact (via email) all group members?
This question have been answer at -  How to loop over a list of Organic Group users using Rules

and at - How to send an email to Drupal 7 Organic Groups Role Members? in here is going to show you how to do it with a custom module.

There other alternatives:

1- You can use views_bulk_operations


  I would think you should be able to do it by adding views_bulk_operations to the mix-- that should give you the ability to email users in a view which you can setup with the criteria you need via filtering.


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2- You can do it with Rules


  Looping over a list can only be done in Rules (i.e. for Drupal 7). 


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