Did the Jews and Christians living under Muslim rule during the Crusades join the European crusaders? Did they remain neutral, or fight against the Crusaders? Or am I wrong to assume they reacted in the same way, and some joined the crusades while others opposed them?
I think you'd be interested in this earlier thread:
Although the initial post focuses on Jews, further down in the thread another user (who has since deleted their account) and I discuss whether there were Byzantine or Oriental Orthodox Christians present in Jerusalem during the Crusades, or if that's a historiographical myth.