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It's used like "ladies and gentlemen" is in English. It's a common expression to hear on public announcements, for example on trains. "Ladies and gentlemen, we will shortly be arriving at..." is Meine Damen und Herren, wir....
So, really, it's just that it's part of the German expression to say meine, whereas it isn't in English. So in this case the best translation should not be strictly word-for-word.
If you say "meine Frauen", you're more than likely saying, "my wives" since "mein Mann" and "meine Frau" translate to "my husband" and "my wife" respectively.
Obviously the good Gentles who are commenting are not members of the SCA (Society for Creative Anachronism) where such language is common. In fact, the battles are being fought in Pennsic War 46, as I type. Camping medievally with 10,000 of your closest friends, bashing your enemies by day then singing and drinking with them by night, bowing over my Lady's hand to reverentially kiss her fingers and buying a handmade plague right to curse your enemies and delight your pets. Believe me, addressing someone as My Lady is not sexist when she has whaled the tar of you in that afternoon's battle, and she will in good fun flutter her eyelashes at you and answer with a curtsy and a "Thank you My Lord.
Another way to look at it: In English we use the word "Madame" (although a bit dated) which comes from the French. Although the French use the same word, it comes from "Ma + dame" (my lady.. also dated in French). In French for multiple women you would say, "Mesdames" (Mes + dames = my ladies) et "messieurs" (mes + sieurs = my gentlemen/sirs)
I am astonished that this is still going on. I could give you a disquisition on the origins of the terms Lady, Gentleman, Mistress (Mrs.), and Master (Mr.), their usages and changes over the years, but Duo is teaching modern day languages, not etymology. There is a word, "gentlewoman or gentlewomen" but it is archaic, at least in the U.S. If someone from one of the other English speaking countries still uses it, then prithee, kind sir or madam, I beg you to enlighten me. But I doubt it. In any case, I can't recall ever reading its usage as a noun of direct address. Duo is wrong, and everyone who translated it as Welcome, Ladies, is correct. Because ladies in this case is used as a direct address, it should be capitalized. Welcome, my Ladies is also correct but is pretty old-fashioned and outside the SCA is just about obsolete.