As for preaching and practicing da`wah, it should be done through two forms: First, through setting a good example for others in morality, faithfulness, kindness, etc. This is a duty on every Muslim individual. Every Muslim should translate the Islamic teachings into practice, and by doing so he will invite others to Islam by means of his practice.
Second, through preaching, writing, and other scholarly approaches. This is a duty on scholars and well-qualified people. This is a communal duty; there should be among Muslims who carry out this duty efficiently. Yet, a Muslim who knows something—even little—about Islam is recommended to deliver it to others.
As for the Tabligh group, it is one of the groups that work in the field in da`wah. People who work in da`wah through the Tabligh, or any other legal way, should be well-qualified to deliver the message of Islam.