No need to recite the Kalimah of Islam ( I bear witness that there is no God but Allah and Muhammad is His Messenger) in front of witnesses. The person must believe in Islam in his heart and accept Islam as his religion. He will be then a Muslim.
Reciting the Kalimah in front of others is good but not necessary condition to become a Muslim. Even if some one believes in Islam between himself and Allah, with out informing any one else, is a Muslim.