You must get an invitation from the administrator of a family account to become a family member of that account.
You can receive the invitation by e-mail or SMS. You can also receive a direct URL link.
Open the invitation link in an Internet browser such as Microsoft Edge, Safari, Google Chrome or Firefox and choose one of the following options:
Create a new user
Log in with an existing user
Of course, if you participate with an existing user, all your files will remain on your user as usual. Your storage space will be changed to fit the size of the family subscription.