Inviting others unto Allah ws the jobs of the Prophets (PBUT) ad the Messegers. And inviting others unto Allah is somehting that the people of the nation are commanded to do. What is the meaning of da'wah? Da'wah literally means: inviting someone else, to invite someone to this Religion- to establish the Tawhid of Allah, to worhsip Him alone, no one else besides Him, to follow His Messenger, and to enter into the fold of Islam. Da'wah unto Allah can also refer to inviting someone to some aspect of this religion.
In order to be someone who invites unto Allah, it is not a condition for one to have complete knowledge. Rather, one who invites unto Allah, invites based on what he knows.
So if someone knows something that is true, then he should invite to it, becasue he knows it to be true based on a proof from the Book of Allah or the Sunnah of His Messenger. Thus the duty of inviting unto Allah is divided into parts. Every single one of you is qualified to be someone who invites unto Allah but only based on what he knows from the truth and from guidance. One should not invite to what one thinks is knowledge or to what one thinks is the Religion. Rather, one should invite to what based on a text from the Qur'an or the Sunnah or on a question that is taken from one of the people of knowledge whose opinions should be taken.