why did cassius wanted brutus to join in the conspiracy

Cassius knows that Brutus is a well-respected and influential man. The conspirators want as many powerful men on their side as they can get so that they can convince the public that they killed Caesar for good reasons. Also, Brutus is Caesar's closest friend, so Caesar would never suspect him of betrayal. Caesar is wary of Cassius, and might suspect that he would plot against him, but he trusts Brutus.