If you are enquiring about a TalkTalk MyAccount then the MyAccount in someone's name can also have a nominated user or users that also have the ability to discuss or manage aspects of the account and include making payments. The advice article is Nominated user
If someone is managing the financial matters of a MyAccount customer then they will need Power of Attorney/Authority to Act
If either doesn't fully cover your enquiry then please let us know.
Email accounts are created in MyAccount and registered in the name of the MyAccount customer even if used by another family member. Managing your email in My Account explains how to manage the email mailboxes.