Using the great tutorial at http://webwash.net/tutorials/log-site-activity-message-and-rules I have been able to set up my Site Activity Log. I now want to extend it to be able to use message_subscribe and message_notify to send users, that are following the organic group, an email with the info. In rules I have been able to add an action Send Message with Message notify. However, I can't seem to set "The recipient of the email" to users that flagged (following) the group. Any insight into how to pull this into the rule would be much appreciated.
There is a Flag action called Fetch users who have flagged [entity]. It will generate a list of users that you can then loop through and perform action on (e.g. send an email to). And since OG groups are entities, it should work (although I have not tested this). Also, beware that this might not scale if you have upwards of 1000 subscribers.