There’s been some talk about an invitation system:
http://buddypress.org/forums/topic.php?id=394
http://buddypress.org/forums/topic.php?id=782
(there’s lots more threads)
But there’s also a need to do confirm actions such as password resets or email changes (both are already handled by MU, I know), or cell phone numbers to receive SMS messages.
The built-in WordPress cron and ajax functions seem to offer a clue about how to create a common API to solve this issue:
Register an action and a function to be called when that action is executed.
The API could also allow you to store data to be sent to that function when it is executed.
