To add the Send E-mail to user action to the workflow, click on the "Send E-mail to user" icon.
To set the parameters, click on the node.
The settings window will appear.
Insert the name of the node.
Select the e-mail template.
Select the users to be sent.
When everything is set, click on the "Confirm" button.
To test the e-mail template, click on the node
and then "Send preview" button.