Invite API

Please look at the discussion under issue 68009.

I don't have the programming skills to do it myself, but here is how I would like an invite API to work:

1) A module (invite, event, organic groups) passes an invitation to the invite API.
2) The invite API determines the invitation's dependencies (perhaps an event invitation requires membership in an organic group, which requires site membership) and whether the user's permissions allow them to be met.
3) If the invitation's dependencies cannot be met, the invite API refuses to create an invitation. If they can be, the invite module presents a list of available methods for sending the invitation. Methods may be provided by other modules and could include e-mail, text message, voice message, or posting to a Web site set up to receive invitations (could allow invitations to be sent to members of other Drupal sites).
4) When a user accepts the invitation, the invite API combines the invitation's dependencies into one process (register user for site, add user to group, and sign user up for event).

Duplicate of issues 29678 and 54947.