Yes is is necessary because although it is a possessive, you don't know which troupe they are talking about. If it had been the name of the theatre company with the possessive, then an article wouldn't be used. If you removed the 'a' it would sound as though the theatre company were called 'West End Troupe'.
New words, one handy idiom, and a 2-minute quiz — delivered to your inbox to keep your streak alive.