This area permits the user to send mail to all members of the group by writing just one mail instead of writing one for each member

The Bulk Mail History link shows all messages that have been sent to the group members
The New Bulk Mail link permits the administrator to create a new message
To create a new bulk mail, do as follows:
Inside the bulk mail portlet click on 
By doing this you will be redirected to a screen like this

Select the members of the group who will receive the message
Click on "Compose Bulk Message"
After writing the content of the message, click "Send"