RegDwight starts out right but, I think, gets confused on a special case.
"I", "we", "he", "she", and "they" are used when the person referred to is the subject of the verb, i.e. the one doing the action. "Me", "us", "him", "her", and "them" are used when the person referred to is the object of the verb, i.e. the one being acted on. So, for example: "I threw the ball to him," but "He threw the ball to me." Or "We saw her" but "She saw us". ("It" is used for both subject and object. "You" is used for both subject and object, singular and plural. Very versatile word there.)
But when the verb is a form of "to be", then the object is called a "predicate nominative" and takes the subject form of the pronoun. The idea, I think, is that when we say "is", nothing is being acted on, we are just expressing identity, so the subject and "object" are equivalent. So the correct usage is, for example, "It is I," NOT "It is me." Or, "The chosen person was she," NOT "The chosen person was her." Many people get confused on this because we're used to using the object form after a verb, and forget that "to be" is special.
All that said, in the example you gave, "us" is correct. The verb is "left". The object of that verb is "us". As this is an object, and the verb is not a form of "to be", then it should use the object form. Don't get confused by the "be" in front of "left" -- that's modifying the tense of the verb, not a stand-alone "to be".