The evidence from the Qur’aan and Sunnah indicates that it is obligatory to call people to Allaah, may He be glorified and exalted, and that this is one of the obligatory duties. The evidence for that is abundant, such as the verses in which Allaah, may He be glorified, says (interpretation of the meaning):

“Let there arise out of you a group of people inviting to all that is good (Islaam), enjoining Al-Ma‘roof (i.e. Islamic Monotheism and all that Islaam orders one to do) and forbidding Al-Munkar (polytheism and disbelief and all that Islaam has forbidden). And it is they who are the successful” [Aal ‘Imraan 3:104]
The scholars have clearly stated that calling people to Allaah, may He be exalted, is a communal obligation (fard kifaayah) with regard to the regions in which the callers live, for every region and area needs the da‘wah (call) and needs people who are active in this field. So it is a communal obligation; if sufficient numbers of people undertake it, the duty is waived from the others and for the others da‘wah becomes a confirmed Sunnah (Sunnah mu’akkadah) and a great righteous deed.
But if the people of a specific area or region do not undertake da‘wah, the burden of sin is incurred by all of them, and it becomes obligatory for all of them; each person must undertake to call people to Islaam, according to his ability. However, generally speaking, with regard to every country, what is required is for there to be a group that is devoted to the mission of calling people to Allaah, may He be glorified and exalted, throughout the world, to convey the message of Allaah and to explain the command of Allaah by all possible means, because the Messenger (blessings and peace of Allaah be upon him) sent out callers, and he sent letters to people and to kings and rulers, calling them to Allaah, may He be glorified and exalted.
[Majmoo‘ Fataawa Ibn Baaz, 1/330 – Shaykh ‘Abd al-‘Azeez ibn Baaz (may Allaah have mercy on him)]