YA from the 1990s about four friends and they attend a Halloween party at one point

[Casey]

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This book was one I read in 1994-1995 for NC's Battle of the Books. In the book there are four friends who grow up together. I think it was three guys and a girl, maybe. At one point in the book, they attend a bonfire costume party. At the end of the book, one of the friends commits suicide.

[Daisy]

Something by Christopher Pike - The Party or another one from this series?

[Kris]

Following up on Daisy's suggestion of The Party (1988) by Christopher Pike:

Google Books preview which includes "The Party" - https://books.google.com/books?id=-NJ... There are different cover images.

Google Books preview of "The Party" with a snippet (keyword) search - https://books.google.ca/books?id=msTr...

[Lobstergirl, au gratin]

Casey left the group. Moving to Abandoned.