Are you talking about Adjest? If so, I don't believe either, Desir or Adjest, announced that she joined Desir's party. Adjest just asked to speak to Desir privately and it escalated to her asking to join. I think it was more of surprise to everyone else because they thought she was in that Blue Moon(I don't know if that's the correct name, I can't remember) party. No one ever questioned what was discussed when they chatted alone so I don't understand how anyone would know that she joined.
But, I could have just missed a page when reading and it was announced previously before. I don't know. (⌒▽⌒)
P. S.
I'm not looking to start an arguement with anyone about this, just replying to a comment. Sorry if I misunderstood the story in anyway.
(AKA, I'm too shy(wimpy) to argue or fight) ٩(๑❛ᴗ❛๑)۶
Lol people be surprised she showed up, but didnt she literally ask to join his team just a few chapters prior or smth?