Is it permissible to go in Tableegh Jamaat for Da'wah work? Are these people on right path? Please guide me.

Praise be to Allah, the Lord of the World; and blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

Jamat-e-Tableegh is a Muslim Jamah known for its large and vital activities in the field of Dawah. The Muslim scholars and callers to Islam have various opinions concerning this Jamah; some of them support it while others criticize it. The moderate opinion is of those who neither condemn it totally nor approve of it totally, but admire its good activities, which agree with Sunnah, but do not agree with those activities which are not from Sunnah. This is the moderate stand and should be adopted by Muslims when evaluating any Muslim group whether it is Jamat-e-Tableegh or another group.
As for going out to make Da'wah, it is permissible in general but fixing a certain period such as forty days or three days, etc. is not proved from Shariah as per our knowledge; for more details please refer to Fatwa:
82024.
Allah knows best.