Go to the Assignments module.
In the Confirmation emails waiting to be sent section, click on Set up and send confirmation emails.
Select the recipients for the assignment email. You have several options:
All pending emails (assigned or declined members)
Only assigned members
Only members whose assignment was declined
Specific members (manually select from the dropdown list)
One or more custom views created in the Members module
Once your selection is made, click Send confirmation emails, and confirm the sending.
Tip: If the email is not received, you have the option to resend it manually.
Follow this article for more information.