‘Ali ibn Abi Taalib's, may Allaah be pleased with him, call to Islam
1-8-2006 | IslamWeb
Question:
When did ‘Ali, may Allah be pleased with him, start preaching, though it may have been secret in the early years? Could you also give some examples of his preaching and who all accepted his call?
Answer:
All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.
The Companions 
started calling to Allaah as soon as they embraced Islam. For instance, when At-Tufayl ibn 'Amr Ad-Dawsi 
embraced Islam, he told the Prophet, sallallaahu ‘alayhi wa sallam, "I will return to my people, and I am indeed obeyed by them, so I will call them to Islam as Allaah may guide them to it." Additionally, Abu Bakr was the first man to embrace Islam, and he called other people to Allaah The Almighty. 'Uthmaan, 'Abdur-Rahmaan ibn 'Awf, Talhah, and others became Muslims thanks to him.
As for 'Ali he was the first boy to become a Muslim, and he was nurtured by the Prophet 
. 'Ali, was nine years old when, he began to receive revelations as a prophet. However, we are not aware of any Companion who embraced Islam due to being called to Islam by 'Ali . Undoubtedly, he was in the company of the Prophet and he never stayed away from him until the latter died. He fought in the cause of Allaah The Almighty and benefited the whole Muslim nation.
Finally, for more benefit, please refer to Fatwa 84903 about the biography of 'Ali .
Allaah Knows best.