When you activate users as an administrator, an invitation email will be sent instantly:
As mentioned in the email the link for the password is only valid for 5 days.
When the user clicks on the link after those 5 days it will no longer work. To be able to setup the password, you or the user can follow the procedure of "Forgot your password".
If your colleague deleted the mail by accident, you can click on the 'Resend invitation mail'-button in the users details and a new mail will be sent:
