“Pirkey Avos says: Al Tischabeir L’Rosho! There is no such thing as achdus with people who deny the Ikkarim such as reform and conservative ‘rabbi’s’ etc. since they have removed themselves from klal Yisroel. Of course it is a Mitzvah to bring back the members of these comgregations to Judaism but their leaders are machti es horabim and off limits.”
Thought you may find it interesting to know the Rebbe made a clear distinction between the reform ‘Rabbi’ as a clergy member, and as a person.
The Rebbe simultaneously encouraged shluchim to mekarev then as individual yidden when they talk to them, or invite them over,because theyre Hashem’s child who deserves to be brought back.
However, simultaneously, we are never to give legitimacy to the false ideologies by sitting with them on a panel of clergymen, entering their temples etc.