In the new TalkTalk Mail the Contacts Group is called a Distribution list. You'll find your previously defined Contacts Groups in the alphabetical Address Book Contacts list with a sub-title of Distribution list.
First, remember that a Distribution list should be no more than 50 recipients. Larger groups should be split up. Sending limits will apply. No more than 50 recipients per hour and no more than 500 recipients a day are the guidelines. More than that and Spam protection blockers will activate temporarily to keep you to the guidelines.
To create a new Distribution List (group email) in new TalkTalk Mail:
To Edit an existing Distribution list
To send an email to a Distribution list