Cassius feels that Brutus would be a useful member of the conspiracy, for more than one reason. First, he is close to Brutus, and feels he will be able to convince him of the value of the idea. He plays upon Brutus' love for Rome, and the ideals of Rome, to convince him that Caesar's behavior is wrong and must be stopped -- something he thinks Brutus already believes. Cassius reminds Brutus that Caesar is just a man, the same as they are, and they should not allow him to set himself up as a king.
Cassius's other reasons for choosing Brutus specifically revolve around Brutus's standing in Rome, with the senate, and with Caesar himself. Brutus is Caesar's "angel," beloved of him, whereas Caesar dislikes Cassius and would therefore be distrustful of any instruction Cassius might give him. Cassius needs Brutus on hand because Brutus will not only know Caesar's movements, but be able to persuade him one way or another to be at the senate on a certain day.