Rabbi Aharon Feldman, Rosh Yeshiva of Ner Israel Rabbinical College, wrote a responsum regarding how to interact with Meshichistim (defined by him as anyone who believes that R Menachem Mendel Schneerson Zichrono Livracha will be resurrected to become the promised Messiah). He writes that they are not considered heretics, and thus their testimony in religious court and their ritual slaughter is valid and they can even count for a Minyan. However, he writes that since their belief is so dangerously wrong, it is forbidden to assist them in publicizing this belief. If one is present when they overtly declare it (such as through the Yechi statement) then one must protest if possible or at least leave the room. Additionally, no one with this belief should be relied upon for religious rulings (psak) as they are certainly lacking in השגה נכונה proper reasoning, and they should not be appointed Rabbis or religious leaders. One should not go to hear them give divrei Torah as their words should be assumed to be in error, and even going to listen can cause others to mistakenly think the wrong beliefs are acceptable.
Rabbi Shlomo Aviner, Rosh Yeshiva of Yeshivat Ateret Kohanim responded to the following question:
Q: Is it permissible to Daven in a Chabad Minyan, where they believe that the Rebbe is the Messiah?
A: Yes. A person who errs is still counted as part of a Minyan (And similarly, Ha-Rav Aharon Yehudah Leib Shteiman [sic] answers this question: "Is it possible to Daven with someone who believes in nonsense?!" Be-Zot Yavo Aharon, p. 371).