Tspence92: To be grammatically correct, it should read: "It is we..." 'Us" is an object pronoun and though commonly used as you say, it's still technically incorrect. Of course we say all the time, "It's me, it's him, it's her, etc." but if you turn it around and start with the predicate NOMINATIVE, you'd clearly see the error: "Me it is, him it is, her it is, etc." Colloquial English is odd for that reason. Say it one way it's "Us", say it another it's "We". Other languages such as German, e.g., maintain the nominative subject case, regardless of work order.
Correct is what people say. The 'nominative argument' just doesn't cut ice. Latin and German do it that way, many other languages (Arabic and Polish for instance, though I can't remember the details) do it differently. And in German you can't even say 'That's I' or 'that's me' - it has to be 'that am I' (das bin ich). Different logic altogether. Yes, colloquial English is weird, but that's because all languages, colloquial or otherwise, seem weird to those who don't speak them.
"Correct is what people say."
Suppose this is true. All language and grammar lessons are consequently unnecessary. Whatever you are saying is correct. You have nothing to learn. Or if you do have anything to learn, it would not be in a classroom, but on the street instead. Because whatever anybody else is saying is correct as well. Why are we wasting our time here, asking about whether this or that expression is right? If you heard someone say it, it's correct! How wonderful is that!
redbrickhouse: You're exaggerating and overstating what I and other users have said. It's not that grammars are unnecessary and should be discarded in favor of whatever is said out there on the street. The point I tried to make is that when what a majority of native speakers say over time becomes so ingrained in the language that it's accepted as natural by that majority, then grammars should reflect that acceptance. Blind adherence to and insistence upon outdated norms will in the end only impede this natural development of a living language such as English.
The "correct" grammar ("it is we") to my ear sounds weird. I think there's a very good reason for this: Most of the time, we is the subject of a sentence which is invariably followed by a verb. When I hear "we", I expect to hear words continuing the sentence - "it is we who do the grunt work of correcting grammatical errors."
"Us" is often found at the end of phrases or sentences. I don't expect to hear some sort of continuation. As the object of a sentence, it completes the thought.
So, using the phrase, "It is us" permits my ear to hear a completed thought, an entire sentence. If it's "It is we", I expect more words, the sentence seems incomplete, even though technically it's not.
This feeling is made stronger if "It's" is used instead of "It is": the sentence "It's we" sounds really, really awkward, while "It's us" sounds just fine. Technically, "It's we" is the grammatically correct choice - but you won't find me saying it.
redbrickhouse: Because that's what's said. That doesn't make it more correct or "It is we" wrong, but living languages change with time and usage and progressive grammars reflect that change. Most speakers say e.g. "It's him" not "It's he" etc. Languages grow and adapt, its users should too. Norwegian e.g. says "Det er meg" (objective case) -- i.e., "It's me" -- while Italian says "Questo sono io." (subject case). Bottom line: Non è meglio, non è peggio, è diverso.