I recently developed a drupal module that is capable of sending invitations for organic groups via a mail. The idea of the module is to send hashed links (like the ones sent when the user has to confirm the account) and when the user will access those links he will be automatically logged in and subscribed into the group.
The idea of the module came to me when I developed an application where the users could invite other users to join their groups, but the links from the mail were something like: og/subscribe/node_id. Now, if the user that got the mail was not already logged in, he would get an access denied, or redirected to the user login page, then he had to login, and the he became a member of the group. This module skips the login process and subscribes the user automatically based on the special token from the link. So this module improves the usability of the subscribing process in the applications where organic groups are used.
The hashed link looks something like: group/node_id/join/user_id/1b110493dd0152f927d2883a1ba23f69 and the invitations can be sent from a url like: og/users/node_id/invite_user and it is shown as a tab near the "List" and "Add Members" tabs for an organic group.
Any feedback from you is welcome! I am working now on debugging and testing. Then, after a stable version I will continue with new features.