Let's your most hated person in the class suddenly popped up joining your group for a planned eating out. Now you are annoyed, and you said:
If I had known you ARE coming along, I wouldn't have gotten here really
Or
If I had known you WERE coming along, I wouldn't have gotten here really
This a 3rd conditional construction, after you applied (If I had) + participle, what is next? ARE or WERE