When you invite a new user to Cyberday, you can deselect "Send the email invitation and allow access". In this case, the user will not be notified of the access to Cyberday. This way, you can first make entries and invite others in when a good number of items have already been assigned to them.
To do this, you need to remember to send an invitation later. You'll separate these users from the "Uninvited" stamp in user management, and you'll see a "Send invite email" link on the right.