When adding users, you can generate an invitation or not. You can send an invitation immediately or later. If you opt for later, you control the time at which the invitation will be sent. When you are ready, click on 'Sending an invitation to all non-registered users' from the administration interface, 'Users' tab.
By non-registered users the system means pending and draft users.
When you click on this link, a window invites you to write an invitation text that will be included in the email.