I don't really understand what "predetermined companions" means, but I don't have and option to summon Shae. She's on my list (like everyone else), but I can't do anything with her.
Thank you all for answer. To be honest - I have read the warning, but... It was a long time before I started this chapter and I didn't remember all that will happen. I have read warning when I was still in class missions after I reached 60 or 70lvl and I just forget to read it again when I finally started KOTFE.