The Radcliffe, Rowling, Grint, & Watson are forgeries. That doesn't speak well for the rest.
the rowling is a fake . Not familiar with the other sigs
Many look signed by the same hand but agree with others that the main names are fakes
It's not out of the realm of possibility to have thirty-plus people sign a book while having some with similar handwriting styles, but more information is probably needed. Does your contact remember who he got it from? Would they be able to share any information? Also, how much did you pay for the book?
Also found a pic of Radcliffe signing a book at one of the premieres, but it looks extremely rushed.