"She does not need us."
In English we usually say, "She doesn't need US," where "us" is the direct object. But we could also say, "As for her, WE are not needed." In the latter case "we" is the subject of the sentence, not the object. If you translate the Japanese sentence into English like that, it becomes clear why you need が instead of を. I see you are also studying Spanish. I struggled (a long time ago) with how to say "like" in Spanish. I kept wanting to say "Yo gusto los libros," (a direct translation from English) instead of the correct way of saying in Spanish, "Me gustan los libros." ("The books are pleasing to me.")
There are also other way to say we in Japanese instead of 私達 僕達/僕等（often heard in various Japanese songs） 俺達/俺等 あたし達（I use アタシ達）
What's the difference in actual meaning between "いりません" and "いらないです"? It doesn't accept the latter, but it's what I'd been told was the normal way to say "I don't need X". My understanding is that the "ません" form can be considered a bit rude/forward (https://www.quora.com/What-is-the-difference-between-%E3%81%BE%E3%81%99-and-%E3%81%A7%E3%81%99-in-Japanese backs me up here)