To be there. I mean, he's. Iron Man. He could be there in like an hour, regardless of where he was. Further, Stark is intimately ingrained in SHIELD in ways nobody else is, right down to providing them with tech. His father basically made Steve into Cap and is the primary reason the agency even exists, so his absence is highly conspicuous. It doesn't really matter, but it's something that's definitely noticeable.

Thor could be anywhere in the 9 realms, and his absence is completely plausible IMO and needs no explanation.

Banner could be hard to find, but apparently SHIELD tracked him fairly well. I suppose his absence is plausible.

Hawkeye though? No excuse whatsoever for why he wasn't a player in this. He's a SHIELD big dog and absolutely should have had a role here. Ditto Coulson, since Hill was there.

That said, Hawkeye remains a somewhat nagging absence. Overall this isn't a big deal despite the discussion we're having though I do think it's obvious and pertinent to ask where Stark and Agents Barton and Coulson were. Coulson can be explained in the series, which is a good thing, but unless the tv explains Barton's whereabouts, that's the only truly glaring ommission from this movie.