it; used to intensify the subject, especially to emphasize that it is the only participant in the predicate
The door itself is quite heavy.
it; A thing as the object of a verb or preposition that also appears as the subject
We; intensifies the subject as the group including the speaker, especially to indicate that no one else satisfies the predicate.
Us; the group including the speaker as the object of a verb or preposition when that group also is the subject.