Why didn't the conspirators want Cicero to join them?
The conspirators do not want to let Cicero in on their plot to assassinate Julius Caesar because Cicero apparently is a poor follower. In Act II, Scene 1, Cassius suggests that Cicero be included. Casca, Cinna, and Metellus support that idea. They believe that he and his "silver hairs / Will purchase us a good / opinion." His positive reputation with the people of Rome will help to support the decision to murder Caesar. The conspirators recognize the inevitable backlash that will follow their killing of such a celebrated man, and they need all of the help they can get to combat the uproar they will cause. However, Brutus protests the inclusion of Cicero in their scheme. He states:
O, name him not; let us not break with him,
For he will never follow anything
That other men begin.
This means that Cicero is bad at following the orders of other men. "He will never follow anything / That other men begin" is another way of saying that Cicero wants to be in charge of everything that he is a part of. His inability to be a follower instead of a leader will make him a bad part of their plan and might cause unnecessary problems in the execution.