Automatically Subscribing Nodes to Organic Groups based on Taxonomy Term
Custom module will help. I would use node_insert() and node_update() although we have used submit handlers for this in the past also. 

Code to add a node to a group is: 

$form['group_audience'][LANGUAGE_NONE]['#default_value'] = $group_audience;


I have read that og_group() can set a node type entity as group content, but the api looked like it changed to us.

The taxonomy_access module might provide another way of thinking of this that suits your needs.