Imam al-Ajurri said:

“At the time of the Prophet (peace be upon him), people came from Arab tribes and distant villages. Small groups from every town and village accepted Islam and learned what was obligatory for them. Then they would return to their tribes and teach them Islam and the faith in accordance with what the Prophet (peace be upon him) had taught them. Moreover, they would teach people what is lawful and what is unlawful. Those who had learned from the Prophet (peace be upon him) would explain to their people that the Prophet (peace be upon him) had told them such and such a thing.

Allah said [approximate translation of the meanings of the verses]:

"The believers do not have to leave all their homes. Why should not a few men from every clan come and learn the religion, that when they return they may warn their people, so that they may be on their guard?" (Surah 9: Repentance, verse 122.)

This indicates, and Allah knows best, that whenever these people came to the Prophet (peace be upon him), they accepted Islam and learned their religion. He (peace be upon him) encouraged them to memorize the Sunnah if they could, so that they would return to their families and teach them what they had learned from him (peace be upon him).