ANSWER:

One must not join them except if he’s a Scholar of sound Aqeedah, with the intention to teach them. 

You as a layman must never join them. 

Kindly refer to the following link(s): 

What is the ruling on joining Jammat at-tabligh. Can one follow them in their tours of dawah without following their da’eef, fabricated and odd hadiths? If someone follows them on their missions, will I be associated as one of them even if i reject their deviance and only uses them for dawah?