How do you address a formal letter to a group of women - i.e. the equivalent of Dear Sirs, for women?
I see no reason to specify gender, and would suggest, in the case that you don't know the people you are writing to:
In the U.S. I don't think it would feel too informal to address letters this way, unless it were a legal matter or there were a contentious, adversarial tone to the communication.
However, if "Dear friends" does feel too informal for you, here are some alternatives:
Good morning [afternoon],
This is a nice trick with a formal group email, and I don't see why it wouldn't work in a business letter format.
Now, since Dear sir or madam is technically addressed to a single person, but it works for a group, you could follow that pattern, and address your letter to a single female person:
Which follows the old-fashioned pattern of
These would only be used if you don't have a name to address the letter to.
If you need to address a letter to a clearly defined group, there are already Q&As about that here; let me know if you want me to hunt one down.