The opening statement of a letter sets the tone of the entire correspondence and lets the reader know whether it concerns a formal business matter or a personal issue. When addressing a letter to three or more people, using the proper plural words helps you present a professional face to the addressees.
Start the address with a salutation, such as "Dear," that is appropriate for the type of letter. Write the names of women first and address them by title. Use "Mrs." for a married woman or "Miss" if the woman is single or you are unsure of her marital status. Use the plural "Mesdames" or "Mmes." if you are addressing more than one woman.
Write the word "and" between each name of the women being addressed--for example, "Dear Mmes. Janet Brown and Megan Smith."
Address the men after women, using the title "Mr."
Use the plural "Messrs." if you are addressing multiple men in the letter. Place an "and" between the names--for example, "Dear Messrs. Tom Johnson and Luke Field."
Place the word "and" between the names of men and women if there is a mix of both genders--for example, "Dear Mrs. Janet Brown and Messrs. Tom Johnson and Luke Field." Finish the address line with a comma.
Always write out the word "and" instead of using "&" when addressing a letter to multiple individuals. Abbreviations in names are acceptable if the person being addressed normally abbreviates his name in other correspondence.
The plural word "Misses" can be used for multiple women who are not married, but the term is less formal.