Why do they need Brutus to join them?

Brutus is needed to join the conspirators against Julius Caesar because of his reputation for honor and integrity. His involvement would lend credibility to their cause and help sway public opinion, as he is well-respected among the Roman populace. Additionally, Brutus's moral standing makes it less likely that the assassination would be viewed as a mere act of treachery, but rather as a necessary step for the greater good of Rome. His participation could unite various factions against tyranny and solidify the conspirators' resolve.