The OG Preapproved module provides a field to allow group administrators to enter a list of email addresses or domains as pre-approved users for that group. If a user creates and account and attempts to join a group in which their email address has been pre approved, they will automatically be approved to the site.
Currently, the only functionality in place is that the new user's status will be set to 1. There is no change to the OG group "subscription" status. So, if you do not have your user settings set to require administrator approval for new accounts, this module won't do anything for you...yet.
Features I would like to incorporate, but have not yet had the time to flesh out:
Allow admins to choose the OG subscription status that the preapproved members will be assigned to.
Hook into the subscription creation method to evaluate new members, not just on user account creation.
Create an automatic assignment when a user creates a new account so they don't have to choose which group to join, they will be automatically assigned based on which groups they have been preapproved to join.