Addressing a business letter to one person is relatively simple. You write the person's name and address using the U.S. Post Office format, and follow up with "Dear Mr./Mrs./Ms." When you need to address multiple recipients in the same business letter, things get more complicated. Depending on the recipients' location, you'll either address each recipient individually or you'll use the "carbon copy" notation – "cc" – at the bottom of the letter.
When addressing multiple recipients in the same organization, there is no need to repeat the address multiple times. Simply write the name and title of each recipient followed by the single company address. It should look something like this:
Ms. Mary Harris, CEO
Mr. Robert Martinez, Director of Facilities
Dr. Philippa Bennett-Price, Finance Director
123 Acme Street
Lexington, KY 40505
Your salutation should then list the names in the same order as the address, followed by a colon (":"), for example "Dear Ms. Harris, Mr. Martinez and Dr. Bennett-Price:" Writing "Dear Mary, Robert and Philippa:" is perfectly fine if you are on first-name terms. It is courteous to send a separate letter and envelope to each person, so print and sign an original copy for each recipient.
When the same letter is to go to multiple recipients at different locations, each person receives an individually-addressed letter. Indicate that you have sent the letter to other people by putting "cc:" at the bottom of the letter beneath the signature line, followed by the names of the other recipients in alphabetical order. "CC" stands for carbon copy, referencing the carbon paper that was used to make additional copies of documents before the invention of the photocopier – today, we use the phrase "courtesy copy." Be sure to revise the "cc:" line in each letter so that each recipient knows who all the other recipients are. Include their addresses if this will be helpful to your recipient.
When you have many recipients such as members of a committee, it may be more appropriate to prepare a single letter addressed to the group, and place a distribution block at the end of the letter. It is acceptable to greet larger groups as a body of people, for example, "Dear Investor Relations" or "Dear Members of the Board." If the letter is being distributed internally within your own organization, it's fine to use the informal greeting "Dear all."
While there is a certain etiquette for addressing letters to multiple recipients, there are no hard-and-fast rules. Your company may have its own style which may or may not follow the traditional standards. Consistency gives a good impression of your brand, so do check that all your employees are following the same style in every business communication.