I usually will open with “Greetings Gentlemen” when I know the gender, obviously in that case it’s men. If they’re women who I’m friendly with, I’ll write “Hi Ladies.” In the example you give where you don’t know the gender, I would just start writing, or if I wanted to be a bit more structured, I’d write “hello” as an opener.
Similar to what @jaytkay recommends, I often use : “Dear <company> Staff, ...”
Also, email is quite a bit less formal than other media so you have a lot of leeway.
As in one person or a group of people? If it’s just one person is say, “Good morning/afternoon.” I work for attorneys, so a professional environment, but even there no one is so formal in an email to use, “Dear Sir/Madam.”