For studies that you have included workers for, once you launch your HIT and it is running you will see on the DashBoard an option to notify workers.
This will give you the opportunity to send all qualified workers an invitation email (via Amazon MTurk). You can craft your message to your taste as shown. Notifications are sent by a TurkPrime background task within 2 minutes after you schedule it.
Our experience is that an invitation email for a well paying HIT will speed up HIT participation rate by a factor of 10 times. A HIT that would normally take 10 days to complete will complete in one day!
You can also notify workers using the Manage Workers | Email Workers menu.
This will allow you to notify workers by specifying their Workers Id as shown here:
Fine Print: Amazon places restrictions on the Workers that may be included or notified, such that they must have taken a HIT with that Requester in the past. You are unable to notify pre-qualified workers who have never completed a HIT for you