If I am addressing a few people who are well known to me, I would generally use: Dear John, Bill, Jack, If I am addressing a lot of people, or people who are not familiar to me, I would go with:
Dear All,
english.stackexchange.com/users/1576/louiser89
If you don't know the recipients' names, I refer you to the other answers.
However, if you do know their names, then I will add that I would actually just write: Dear John, Dear Jack, I have been using this formula for more than a decade.
So far, nobody has complained.
For me, this approach has quite a few advantages.
First of all, it is more personal than simply "Greetings" or "Hi all".
Secondly, if one of those people is the main recipient and the other is CC, or if one of them has a higher rank (say, he is the boss of the other), I can reflect that in my address by mentioning him first.
Lastly, this formula can be easily adapted to many situations.
If some (or all) of the recipients are female I simply write:
Dear John, Dear Jane, If I need to make it more formal, I simply write:
Dear Mr. Smith, Dear Ms. Black,
And I can easily extend it to three or even four people: Dear Mr. Smith, Dear Ms. Black, Dear Mr. Blunt,
Now, when I am addressing more than four people, I often do use something like "Greetings", "Hi everybody", "Dear clients", "Dear colleagues".
However, even then I sometimes set the most important recipient apart by writing:
Dear Prof. Black, Dear colleagues, or Dear Mr. President, Dear Members of the Committee,
english.stackexchange.com/users/300/RegDwigнt
I rarely begin emails with a salutation.
If I do, it is usually just the name - I have never transferred the pointless 'Dear' to emails.
If I wanted to put one in a group email, I suppose I would start with a word like "friends", or "people", or "folks", depending on the context and formality.
english.stackexchange.com/users/547/Colin Fine
It's kind of a carryover from my medievalist hobby, but I really like using Greetings, or even Greetings! for emails to a group of people.
It's also really handy for addressing a single person when you're unsure of that person's gender and/or title.
english.stackexchange.com/users/1547/Marthaª
If you are working together on a project, then I rather like Dear team
This can be appropriate even if the recipients include people from both your company and the client.
It's a subtle reminder that you are all working towards a common goal.
More generally, I'd just use Dear all, or just All,
english.stackexchange.com/users/1571/francois
"Sirs" is just completely wrong.
If you are addressing more than one man, it would be "Gentlemen".
english.stackexchange.com/users/6764/Kevin
Email is still sufficiently young that formal conventions have not emerged.
I find it useful to imagine myself in the mind of the recipient and ask what information I need to give them in the greeting.
If the email is really to be sent with equal priority to a number of recipients and they regard themselves as part of a group (fellow developers for example), then I would use Dear All, If, actually, the email is a mailshot to a number of individuals who don't know each other, then its an individual email.
Dear Sir, or possibly Dear Sir (or Madam), Here, I prefer to use the convention that Sir is neuter.
And lastly, if an email is really to a single individual, but with a few more individuals copied in (such as a question to an individual developer, but where I want the email trail to be visible to the group), then I would use Dear Bob (Alice, Charlie, Dilbert)
The last one is not a convention I have seen elsewhere, but I find it works for me.
english.stackexchange.com/users/1565/Salm
I have been using, "Hello to all".
It seems to work well for both male and female recipients and is formal enough to use in a business setting.
english.stackexchange.com/users/19692/Marie
If it is a formal letter, then you can use 'Dear Sirs' but if it is informal, simply 'Hi' is fine. '
Hi' addresses either one person or many, the same as 'you' refers to one person or many.
I would not ever use 'Gentlemen', but I am from Australia where our English is more informal.
english.stackexchange.com/users/115832/Irene Hogan
I usually just use the words
Hi All,
english.stackexchange.com/users/64978/Eli
You can also use gents to refer to many recipients.
english.stackexchange.com/users/8078/Raul Romero
I agree with louiser89 and I will add that - even though many people or perhaps most people don't think that "Dear" is appropriate on emails - I will continue to use "Dear" or, in my language, "Caro (Cara)".
It is not only because we have always used it in letters and in commercial correspondence; it is not just traditions.
"Dear" is a nice word, it is gentle and polite.
It is not a word that could ever offend anybody.
And it is certainly not formal, unless you consider formal the people you love or that you consider your friends, or the people you like.
So why not use it also in emails?
How can something nice become formal or old fashioned or, worse, strongly disliked?
Of course we should never consider it obligatory, but why be against it?
Dear Friends, don't be too angry with me: English is not my language, but it is a very "dear" language to me for many reasons.
english.stackexchange.com/users/126334/alsa
How do you greet multiple recipients in an e-mail?  Assuming they're both male, I just use "Sirs", but it seems a bit informal.
How do you greet multiple recipients in an e-mail?
english.stackexchange.com
<email><greetings>
english.stackexchange.com/questions/3700
english.stackexchange.com/users/1269/Jonn
There are many different ways of addressing a group in an e-mail. Depending on how formal the message is, you could start with "Dear All", "Dear Sirs" or "Gentlemen". If you're working on a project together, you could simply write "Dear team". However, you may just want to write all of their names individually if you know them. There are numerous and highly subjective suggestions as how to address people on email. There is the option of using "Dear" and variant uses of "Dear" depending on the familiarity with the recipient/s with the option of adding or not their first name. Some people disagree with using "Dear", but others think it is a gentle and polite word. Others use other generic forms of salutation such as "Greetings", whereas some do not use any salutation at all. It is suggested that as email is a recent form of communication, no standardised salutations have emerged. However, some consider"sirs" to be incorrect in this context whereas "Gentlemen" would be more appropriate, even though Australians feel "Gentlemen" is too informal and "gents" could be used. On the other hand, some believe "Dear Sirs" is correct in a formal setting. Others use "Hi" and "Hello to all" as other suggestions to greet a group of people.
There are many different ways of addressing a group in an e-mail. Depending on how formal the message is, you could start with "Dear All", "Dear Sirs" or "Gentlemen". If you're working on a project together, you could simply write "Dear team". However, you may just want to write all of their names individually if you know them. Email is still sufficiently young that formal conventions have not emerged. If you  know their names, I would actually just write:   Dear John, Dear Jack. A more general greeing is to just use   Dear all,   or just   All or hello to all.  Some suggest not starting emails with a salutation.
Suggestions include Dear all; Gentlemen; Hello to all; Hi; and Dear team. Alternatively, you could greet the people individually, as in: Dear John, Dear Jack There are many different ways of addressing a group in an e-mail. Depending on how formal the message is, you could start with "Dear All", "Dear Sirs" or "Gentlemen". If you're working on a project together, you could simply write "Dear team". However, you may just want to write all of their names individually if you know them.
Use of "Dear" and variant uses of "Dear" Personal style greetings used for emails to a group, which would depend on context. Not use any greeting form at all.
If you  know their names, I would actually just write:   Dear John, Dear Jack. Some suggest not starting emails with a salutation. A more general greeing is to just use   Dear all,   or just   All or hello to all.   Email is still sufficiently young that formal conventions have not emerged.
You could just write all their names individually. Some possibilities for addressing a group are "Dear All", "Dear Sirs" or "Gentlemen", depending on who you're writing to and how formal you want to be. If you are working together on a project, you could address them as "team".