More Info
When in Contact mode you will see any existing Contact Groups in the left hand window. The default Contact Group is the All group, containing all Companies and Contacts. You can create a new Contact Group by clicking Edit, New, New Contact Group. It is possible to drag and drop these groups to create the required nested structure.
Note that this technique for sending to a list shows the complete list of recipients in the headers of the message.
Detail
To send a message to a Contact Group, simply create your message and click the To button. In the top right of the Email Address Book dialog box select Contact Groups. Select the required Contact Group and click Add. Repeat for other Contact Groups who are to receive this message. Click OK.
Note: Sending a message to a Contact Group will only send the message to the contacts in that group, and not to the contacts in any sub groups. These would need to be explicitly selected.