Everybody except Marcus willingly chose to participate in this. The ICC put the word worldwide that it was looking for volunteers to undertake this quest. You can't force people because the world is so dangerous that it's basically a death sentence. Whatever the reasons, all of your characters are willing volunteers. 

Yes, you would know to meet at Armistice. That's what would have been spread by messengers (messengers being royal knights). 

You can also just have already been in Armistice at the time though. If that's the case, you would have heard about it since the ICC obviously calls that city their home.