Da'wah - inviting people towards Islam - is one of the obligations a Muslim must perform. Dawah includes clarifying the misconceptions, presenting the true picture of Islam, inviting to righteousness, forbidding evil, and help the society maintain peace and harmony. It never involves coercion, undue influence, fraud, or disrespect.
As a Muslim one must perform this duty of clarifying and presenting the message of Islam to those who are misinformed, dis-informed, or uninformed. The Quran says, "Invite (all) to the way of your Lord with wisdom and fair preaching and argue with them in ways that are best and most gracious." (16:125).
The 14 principles of Dawah: Shaykh Muhammad bin Saleh Al-Uthaymin
Source: The Islamic Awakening (download the whole book (pdf))