There are 2 ways to send assignment notifications from within tasks:
If you assign a resource to a task while creating the task, when you click Save in the New Task panel, a popup appears asking if you want to send the assignment notification now (if you have the resource’s email address on file in RB). Click Yes. Then select notification options and send the notice. See Notifying resources individually.
If you clicked No in the popup, you can send the notification as part of a batch, or when editing the job. You can also use this method to resend an assignment notification:
When a resource acknowledges an assignment, enter that information in the assigned task’s Edit Task panel of the relevant job. See Logging resources’ acknowledgements.