Great Daee - Abu Dharr (RA)

بِسۡمِ ٱللَّهِ ٱلرَّحۡمَٰنِ ٱلرَّحِيمِ

Abu Dharr (RA) continued, "I left and went back to my people. My brother came up to me and asked, 'What have you done?' I told him that I had become a Muslim and that I believed in the truth of Muhammad's (SA) teachings. 'I am not averse to your religion. In fact, I am also now a Muslim and a believer,' he replied. We both went to our mother then and invited her to Islam. 'I do not have any dislike for your religion. I accept Islam also' she said.

Abu Dharr (RA) had not spent much time in Makkah and he probably had not learnt much about the new religion, as the revelations had only just commenced. But from that day, this family of believers went out tirelessly inviting the tribe of Ghifar to Allah (SWT), and did not flinch from their purpose. Eventually, a large number became Muslims and the congregational prayer was instituted among them. This family's actions were according to the saying of Prophet Muhammad (SA) that we should convey to others, even if all we know is a single ayat. The lesson here is that our job is to invite others to the monotheism of Islam with whatever we know. Hidayah (guidance) is in the hands of Allah (SWT) and the choice of the individual.

The rival tribe of Ghifar was Aslam. When they heard that Ghifar had accepted Islam they also when to the Messenger of Allah (SA) in Medina and accepted Islam. Thus, the acceptance of Islam by a whole tribe began with the efforts of a single man.
{See Sahih Muslim 2473 a}
The Supplication Of The Prophet (SAW) For Ghifar And Aslam
 Abu Dharr reported Allah's Messenger (ﷺ) as saying:
Go to your people and say that the Messenger of Allah (ﷺ) says: Ghifar (is a tribe) to whom Allah granted pardon, and Aslam (is the tribe) to whom Allah granted safety.  Sahih Muslim 2514

We must not underestimate the value of any good deed, no matter how trivial it may seem.
Abu Dharr (RA) probably did not think that his whole tribe and their rival tribe would accept Islam because of his acceptance of Islam, but he preached to his tribe anyway. We should not miss any opportunity to do a good deed, even if it is as small as smiling for another Muslim; we never know, but Allah (SWT) might love that small deed and give barakah (blessing) in it, and make it a source of our maghfirah (forgiveness).