Having read the book, you should know that Percy is a perfectprefect at Hogwarts school and is portrayed as a pompous guy, unlike the rest of his family. George, his brother, is telling Percy that although he (Percy) supposedly wants to sit with the other prefects (as he usually does), his brothers (George, Fred, and Ron) expect him to sit with them this time.
"Either" in this case means "also" or "in addition to what I just said". It does not suggest that someone else is not going to sit with the prefects.
It is a likely interpretation that George is making fun of his brother.