In the new TalkTalk Mail the Contacts Group Email Addresses 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.
To create a new Distribution List (for group email) in new TalkTalk Mail:
To Edit an existing Distribution list
Tip: Keep each Distribution list to no more than 50 individual contact addresses. This is to make sure the spam detection doesn't get triggered by the sending to more than 50 recipients per hour. If you have a group of more than 50 recipients split into several Distribution lists then spread the sending out over the day.