The complete subject of the sentence is the NP (noun phrase) "the organizers of the event". The head of this NP is the noun "organizers" and this noun is postmodified by the PP (prepositional phrase) "of the event".
In order to identify the subject of a sentence, you should ask "who were unhapppy with the turnout?". In this way, you'll get the who or what the sentence is about, that is, the subject of the sentence. In this case, the answer to that question is "the organizers of the event".
