Allow multiple invitations (from different users) to a single email address.

Hi,

I was wondering if you ever thought about allowing multiple invites (from different users) to go to a single email address (invitee). I've had a few situations come up where this feature would be useful.

I've seen that the database uses the invitee's email for its primary key, which would obviously need refactoring if such a feature would be included.

thanks!