Lei CI sarebbe stata is different from "lei sarebbe stata" (which wouldn't mean anything). It generally means that she would have been there/she would have agreed, but I wouldn't use this sentence too much, as it is generally used in a very sexual way.. (something like "she would have agreed to do something sexual with me")
This sentence adds confusion. Would've been nice if when we were first exposed to the Italian "Ci" we would've been given practice sentences that used "Ci" to mean "there". Instead they wait until we are learning a totally different lesson to expose us to the different usage of the Italian "Ci". This seems to be a common theme with the Italian course and it does nothing but frustrate the learners.