What is the masalah for participating in bid'ah with the intention of reforming?

The Hadith has severely condemned all bid'ah practices. Rasulullah (Sallallahu Alayhi Wasallam) said, "Every bid'ah practice (innovation) is misguidance, and every misguidance leads one to the fire of jahannum." Hence, it is not permissible for one to participate in such a gathering.

The good intention that one has of reforming the people after joining them and winning them over will not justify the sin that one has committed of being present in such an impermissible gathering. Rasulullah (Sallallahu Alayhi Wasallam) said, "There is no obedience to the creation if it involves the disobedience of Allah Ta’ala."

Hence, if one is aware that bid'ah practices and haraam is going to take place in a certain gathering, one should not attend such a gathering.

And Allah Ta'ala (الله تعالى) knows best.