You could go with "Friends:" or write "hey everybody!"
The salutation itself ("Dear Ms. Jones") should not include your return address. On a business letter, the return address can go in the upper right corner of the cover letter. The salutation goes below the delivery address.
Usually people sounds either mildly jocular or very odd as a mode of address.
Either you write out the same letter multiple times with different addresses, or you write the letter, leave space for the address, photocopy it however many times you want and write in the different addresses later. That or e-mail.
To address multiple doctors, simply pluralize the abbreviation for doctor. For example, you would write "Dear Drs. John, Bob, and Sara".
Salutation/mixed function
Dear Counselors:
Gentlemen:
Greetings!
salutation
recipient's address and a salutation
Dear Messrs. [Last names}
2