As we all know, Jews don't agree, pretty much on anything. Nevertheless, despite these disagreements, there's a basic, reluctant willingness to sit together at the same table on Jewish issues. We all consider ourselves part of the same community, for better or worse. All the different groups and factions are pretty much represented in international Jewish bodies and in the Israeli Knesset. At the end of the day, we acknowledge that these other factions are, for better or worse, Jewish factions.
I can think of two, and only two, exceptions that I can think of - groups of Jews that are so thoroughly and unanimously rejected by the rest of the Jewish community, that, though Jewish by birth and supposedly representing 'Jewish' factions, they are simply not given a seat at the Jewish table. Across the board.
The two groups are:
a. Messianic Jews
b. Rabid anti-Zionists like Neturei Karta (and I'd lump Holocaust-denying Jews in with this group as well)
What makes these guys so special?