In Act 2, Scene 1, Brutus meets with his various co-conspirators. Cassius suggests asking Cicero to join (line 152). Casca, Cinna, and Metellus all jump to agree with Cassius, and Metellus argues (lines 156-61) that
...his silver hairs
Will purchase us a good opinion
And buy men’s voices to commend our deeds.
It shall be said his judgment ruled our hands.
Our youths and wildness shall no whit appear,
But all be buried in his gravity.
Metellus's argument would have made good sense to many members of a Shakespearean audience. Not only were Cicero's Latin texts--where he eloquently argued for liberty and a moral society--central to the standard liberal education of Shakespeare's time, but he was known to history as a foe of tyranny (he unmasked the Catiliniarian conspiracy while he was Consul) and the dignified Pater patriae of the Roman Republic.
However, Brutus is not convinced (lines 162-4):
O, name him not! Let us not break with him,
For he will never follow anything
That other men begin.
Let's consider two ways to think about these lines. The first is to take them at face value: Brutus disapproves of including Cicero because Cicero is dangerously arrogant and either will refuse to join (which then makes him a possible source for an information leak--he could even possibly reveal the plan to Caesar) or will only agree to join if he can take charge of the conspiracy and get the credit for deposing another would-be-tyrant.
However, we can also interpret these lines as a wry joke that would have certainly resonated with some of the most educated members of the audience. While Cicero was generally considered the zenith of Latin rhetorical eloquence, a model statesman, and a pillar of dignity, even those who admired his example had to admit that his writings and speeches were often quite self-aggrandizing. For example, after the Catilinarian conspiracy, Cicero never missed a chance in a speech or a written treatise to remind everyone that he had been the one to foil it. If we take these lines as a wry joke, Brutus is rejecting Cicero largely because he thinks Cicero is an old windbag who is more interested in being able to toot his own horn than in doing the work necessary for the conspiracy to succeed.