this is from issue discussion on the ghost repository .
@mcoolidge @reustonium . depending on how critical it is (and if you have access to the database backing ghost), you can UPDATE invites SET status='sent' (there is a check that the message has been sent) and then SELECT email,token FROM invites to get the token that would have been included in the url.
Then you can insert the token into /ghost/#/signup// where token is the token from the database less the = at the end.
This has worked for me at least.