It just so happens that I just finished building a Drupal system that stores content until a user with a particular email address logs into a site, then shows it to them.

Why don't you want the users to join an Organic Group? They don't have to explicitly join: you can have it happen invisibly at signup time. You can write code that intercepts the user as they register (or, better, just after they click an emailed link to confirm their email address), notices that their email is on a list of "users that should be added to group X when they show up", and adds them to the group. Then it's just an, um, "simple" matter of using the permissions system. (Which can, indeed, be a little hairy. But not so hairy that lots of people haven't used it.)

Of course, like so many Drupal problems, this will require some custom code...