In my view, the simplest and most elegant solution is to start off simply with
Dear ladies and gentlemen,
having made sure to include the names of all the recipients in the postal address block pertaining to the addressees (if you are sending everyone a paper copy of the letter), so that all of them are able to see who else is covered by your salutation.
If it's an email, you can use the same formula, except that you would include everybody's name in the To: field rather than as part of a postal address block. (Just take care not to insert the address of anyone you are writing to directly, rather than merely sending a copy of the email to, in the Cc: field — or worse still, in the Bcc: field.)
Just make sure you include everyone's actual name in the To: field, not just their email address (e.g. Natasha Smith <nattysmith@aol.com>), so that they all know the real identities of the other people you are sending the letter to (unless it is completely obvious from their email address who they are, e.g. Zebediah.Brown@acmecorp.com rather than an address that reads more like whosyourdaddy7@funnyfarm.co.uk).
Addressing everybody in the form of a list of names ("Dear Ms Black, Ms Brown, Mr Green and Mr White") is not a terrible solution; it's a bit clumsy, but at least you can be sure that no-one will feel ignored or otherwise disrespected.
However, companies often have their own preferred ways of doing things, so you may want to check what the standard mode of address in the target organization is (if there is one at all) by first calling their PR department or equivalent. But if you're still in doubt, actually addressing people by name is the safest approach, because it is the least likely to generate offence.
HelloorGood morningsounds informal. – Arsen Y.M. Feb 18 '15 at 08:40