Retake invite for 621 and re-registration invite for 623 are both scheduled to be sent out on March 22. The invitations will not be sent out broadly until 72 hours after that point so re-takers and re-registering peeps get a seat if desired. For the background on this, you can read this post. Please note that we are not accepting new candidates for the beta exams.
If you are one of the customers who took 621 or had registered for 623, set your spam filter to accept mail from email@example.com so you get your invitation (but dont send mail there because it is an unmonitored alias...).