Post function to notify specified group, when a field has been set in the transitionTo add Assign to a user in a role validator, go to the transition of your workflow, switch to Post Functions tab and click on Add post function link.
Select JEP - Notify when a field has been set and click Add button
Select the field, project role, and fill subject/body of the email template
Click Add button and that's it.
Do not forget to publish the workflow!