I've looked around for this but maybe I'm missing it.

When a user signs up other people for a node and puts the other person's email address in I would like to be able to require the new email to have an account before it is signed up. If the an account already exists for the email then they pass through. Otherwise, the person is prompted to fill in the info for a new account.

Seems like it should be doable but I just can't find it.

Thanks!