Notify inviter when invitee creates account

[root]

Hosts would like a way to know when the users they invite have created accounts.

For "send invitation" recipients, the user could be "identified" by the email address to which the invitation was sent. If the recipient manually chooses (and verifies) a different email address, then we can leave that information out of the 'announcement' to the host who invited him or her.

When announcing account-creation for "create invitation" recipients, we can either: a) not worry about identifying the particular invitation to which it applies; or b) match the chosen username with a particular invitation based on the date/time-stamp on which the invitation was created.

We can decide whether or not this mechanism is appropriate for powerusers (and regular users with "invite tokens") as well.

[Chris (poser)]

There is some concern about storing the full graph of user-relationships in order to make these announcements...even if we don't directly expose the information in any of the interfaces. We should more clearly express the desired or presumed security properties that guide decisions like this.