When your letter is to more than one person, write out all of their names separately, separating them with commas. For example, "Dear Mr. Hobbes, Ms. Luxe, and Mr. Hopman." For married couples, if one person in the couple has changed his or her name, you only need to use the last name once. For instance, "Dear Mr. and Mrs. Smith."
Since your signature is likely to be difficult to read, type your name down a couple of lines so that your reader knows who is writing to them. Enclosures: (Below signature block) If you have included any further material _ a business card, a brochure, a price sheet, whatever _ list the number of items and describe each. Finally, proofread, proofread, and proofread again. While a friend might excuse even the worst spelling and grammar, a business letter is often your first (and maybe only) chance to make an impression _ it needs to be impeccable.