Actually, Antony only appears to befriend the conspirators after Caesar's assassination. Antony was close to Caesar, both politically and in a social sense.
Prior to Caesar's assassination, at a meeting of the conspirators at Brutus' house, Cassius suggests that, in addition to killing Caesar, the conspirators take out Antony as well, thereby eliminating a potential future political threat. Brutus refuses to take part is killing Antony (a poor decision, as it turns out later).
After Caesar's assassination, Antony sends a messenger to the conspirators for safe passage to the [scene of the crime]. Upon his arrival, he is surrounded by the conspirators that murdered his beloved Caesar. Appearing to join them was most likely for survival purposes.
He wants to avenge Caesar's death, but he is outnumbered.
Although Cassius expresses outrage at Antony's request to speak at Caesar's funeral, Brutus allows it (another poor decision), but Antony later appeals to the crowd's emotions by listing all Caesar--whom the conspirators claimed was "ambitious"--had left to the people. This enraged the crowd to the point that it wanted vengeance, blood, the death of the conspirators.