Send an email using velocity script.

This can't be done purely from a widget. The best approach for this is via a plugin which handles RoleUsers events and raises a notification. Besides the fact that this isn't possible purely via widget code, other benefits of this approach are that the notification would be triggered no matter how the role is applied (i.e. through the API, not just through the widget) and the notification would be received by the user in whatever way they have configured (which can be more than just email).

I'd recommend reviewing the following:

• Introduction to plugins
• Handling events via plugins
• Raising notifications from plugins
• RoleUsers In-Process Events

Creating plugin enables sending mails to each and every role .

This can be done by creating  a rule

when user is assigned a particular role then send email