Sentence:  Mrs. Falconer cannot well avoid asking you to some of her entertainments, and it will be pleasant to you to know <WSD>who's who</WSD> beforehand. 
Meaning of the word marked with <WSD> tags:  The identities of specific people, understood in terms of such distinguishing characteristics as their backgrounds, prominence, achievements, jobs, etc., as a basis for comparing them and especially as a basis for ranking them within a social group.