This question came to me because I am writing a personal thank-you letter to a friend's family who hosted me in their home over the holidays. What is the proper way to address the entire family? For example, in English, I would use "To the Ito Family". Beyond this, it would be good to compile a list of typical Japanese openings and closings (salutations and valedictions) used in written correspondence.
For letters to a person, affix it with 様. For Itou family, it would be イトウ(御)一家様. For letters to a company or institution, affix it with 御中.