Group followers

I'm interested in providing group admins with a button to allow non-group members to follow groups (i.e., receive notifications of new public content in the group). This will be especially useful for closed but publicly visible groups, which seems to be increasingly popular on my site. These groups want to have private work areas and also provide public awareness and access to certain pages and files within the group. I haven't found any existing plugins or discussions about creating something like this that allows non-group members to elect to recieve group notifications.

Any suggestions on modifying an existing plugin to provide this capability, or other resources I should look into?