In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
Brother in Islam, we are greatly impressed by your question for it is related to the affairs of the new Muslims. We seize the chance to earnestly implore Allah from the depths of our hearts to lead all perplexed men and women to the light of Islam, the true religion of Allah. We welcome all our new Muslim brothers and sisters to the fold of Islam.
Coming to the point of making Shahadah in public, we would like to draw your attention to the fact that Islam trains Muslims that pubic and secret deeds are known to Allah. Therefore, making Shahadah in presence of witnesses is not an essential condition, though it is preferable to put the official arrangements in order. Islam, as a matter of fact, lays more emphasis on conviction on the part of the person who embraces Islam. This is what really matters, as there should be no compulsion in religion.
Focusing more on the question in point, Sheikh Ahmad Kutty, a senior lecturer and an Islamic scholar at the Islamic Institute of Toronto, Ontario, Canada, states,
Strictly speaking, it is not at all considered an essential requirement for witnesses to be present when a person is embracing Islam; thus if a person declares Shahadah (testimony of faith) between himself and Allah, he is considered a Muslim in the sight of Allah. But in order for him to be recognized as a member of the Muslim community, he must declare the same in presence of two witnesses or before an Imam of a mosque who has been duly authorized to issue a certificate indicating the same.
It is, therefore, highly recommended for anyone embracing Islam to do so in the presence of persons authorized to issue certificates as he/she may be called upon to produce them while planning to go for Hajj or in case of any question concerning his Islamic identity. The Prophet (peace and blessings be upon him) taught us that while doing things, we should try to do them as professionally and efficiently as possible.
Allah Almighty knows best.
Excerpted, with slight modifications, from: http://www.islam.ca