Yes, Chinese often omits subjects and objects where they must be included in English. "You don't need to introduce" is not a complete sentence in English. "Her" or "me" could also be correct, but as we do not know the context, a more general "us" is probably best.
Also English can omit that object!
"You don't need to introduce, I know her." should be accepted as well and of course "gei women" could be added in the Chinese sentence too... - It's just a bug in this cours, if the sentence is not accepted without "us"!
The "us" comes from I and her making 2 people you do not need to introduce us to each other, so Us is perfectly correct. This does not mean that 'me' or 'her' instead of the us is wrong. I also think is should accept I know her, you don't need to introduce us.
You don't need to introduce me should also be accepted as we would say this in English. For "You don't need to introduce us" it should finish "We know each other." Otherwise, both answers should be accepted. Thank you