It actually all depends to whom you're writing. If it is a group associated together because of an organization or club, etc., then "Dear Women's Club of Podunk" is appropriate. If it is an informal memo, then simply put "To:" and list each name, preferably on a separate line. If you're sending one letter to multiple people simply to get out of writing individual notes then don't do it - send individual letters and create a Merge list in your word processing program so that each person gets a separate letter, even if the letters all say exactly the same thing.
Normally, that would be ladies.
Greetings!
Dear Counselors:
Dear Counselors,
The abbreviation becomes plural - Drs. Smith and Jones.
When greeting multiple women in person I would say "Hello ladies".
"Dear" is what people use it nowadays.
What is the proper salutation for a register of wills, e.g. Honorable John White?
You could go with "Friends:" or write "hey everybody!"
the correct salutation for two men is Messrs
Hello.
Your Honor
Dean actually is the proper salutation. One could say: Dean Redmond, would you please sign this form? Alternatively you could just use the Dr. salutation or the Professor salutation if those are appropriate, which they generally are.