Mandatory Group / Autosubscribe in Drupal 7
You can use rules to do this (As OG7 uses rules for everything else).


Create new Rule
Add New Action
React on event: After saving new content
Add condition
Data Selector node:author
Choose whatever role is admin
Add Action
Send email to all users of role
Select whatever roles
Fill in your message with applicable tokens


You can also check out the notifications and Messaging framework or the new Newsletter module for D7. These are better for allowing users to control which notifications they receive and can subscribe and unsubscribe themselves.